A bit of a lurker here, but inspired by the work on this thread, and using the Moonlight launcher as a base, I've created a Steamlink version.
I've only tested it with Libreelec 10/11 and the Raspberry Pi 4 so far, so any feedback on other setups is more than welcome.Steamlink launcher - https://github.com/meekys/plugin.program.steamlink
Awesome job, but yep it's a shame that the Steamlink app for RPi is proprietary to the hardware, as it'd be a wonderful addition to Moonlight on general ARM SBC's (and especially handhelds).
I tried it in December, there was no problem then, but now it doesn't work.
Has anything changed in your environment since December when it was working?
I don't know after all this time.
Updated LibreELEC?
Yes. I have now installed the current 11.0.6 but the error still exists.
Different raspberry pi? (3 vs 4)
Nope. I don't have an RPi3, so it would be difficult to replace it. 
I'm sending a log excerpt for this, if that helps:
Feb 19 06:28:39 RPi4 systemd[1]: Started run-rf5fa745770994f8eb9bb46b8546e6f99.service.
Feb 19 06:28:39 RPi4 kodi.sh[1462]: Running as unit: run-rf5fa745770994f8eb9bb46b8546e6f99.service
Feb 19 06:28:39 RPi4 bash[1466]: Platform 'rpi' running 'libreelec' detected...
Feb 19 06:28:39 RPi4 bash[1466]: Adding udev overlay
Feb 19 06:28:39 RPi4 systemd-udevd[355]: /usr/lib/udev/rules.d/50-udev-default.rules:46 Unknown group 'sgx', ignoring
Feb 19 06:28:39 RPi4 systemd-udevd[355]: /usr/lib/udev/rules.d/50-udev-default.rules:47 Unknown group 'sgx', ignoring
Feb 19 06:28:39 RPi4 systemd-udevd[355]: /usr/lib/udev/rules.d/56-steamlink.rules:2 Unknown group 'plugdev', ignoring
Feb 19 06:28:39 RPi4 bash[1466]: Using Kodi hooks for libreelec...
Feb 19 06:28:39 RPi4 systemd-logind[380]: Watching system buttons on /dev/input/event4 (gpio_ir_recv)
Feb 19 06:28:39 RPi4 (udev-worker)[1485]: eth0: Process '/usr/sbin/ethtool -s eth0 wol g' failed with exit code 75.
Feb 19 06:28:39 RPi4 (udev-worker)[1492]: 0000:01:00.0: Process '/usr/bin/sh -c 'echo disabled > /sys/devices/platform/scb/fd500000.pcie/pci0000:00/0000:00:00.0/0000:01:00.0/power/wakeup'' failed with exit code 1.
Feb 19 06:28:39 RPi4 systemd[1]: Stopping kodi.service...
Feb 19 06:28:39 RPi4 (udev-worker)[1482]: event1: Failed to call EVIOCSKEYCODE with scan code 0x70006, and key code 180: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1482]: event1: Failed to call EVIOCSKEYCODE with scan code 0x7000c, and key code 179: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1482]: event1: Failed to call EVIOCSKEYCODE with scan code 0x7002d, and key code 114: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1482]: event1: Failed to call EVIOCSKEYCODE with scan code 0x7002e, and key code 115: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1487]: event2: Failed to call EVIOCSKEYCODE with scan code 0x70006, and key code 180: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1487]: event2: Failed to call EVIOCSKEYCODE with scan code 0x7000c, and key code 179: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1487]: event2: Failed to call EVIOCSKEYCODE with scan code 0x7002d, and key code 114: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1487]: event2: Failed to call EVIOCSKEYCODE with scan code 0x7002e, and key code 115: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1485]: event3: Failed to call EVIOCSKEYCODE with scan code 0x70006, and key code 180: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1485]: event3: Failed to call EVIOCSKEYCODE with scan code 0x7000c, and key code 179: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1485]: event3: Failed to call EVIOCSKEYCODE with scan code 0x7002d, and key code 114: Invalid argument
Feb 19 06:28:39 RPi4 (udev-worker)[1485]: event3: Failed to call EVIOCSKEYCODE with scan code 0x7002e, and key code 115: Invalid argument
Feb 19 06:28:39 RPi4 systemd-logind[380]: Watching system buttons on /dev/input/event2 (OSMC Remote Controller USB Keyboard Mouse Consumer Control)
Feb 19 06:28:39 RPi4 systemd-logind[380]: Watching system buttons on /dev/input/event3 (OSMC Remote Controller USB Keyboard Mouse System Control)
Feb 19 06:28:39 RPi4 systemd-logind[380]: Watching system buttons on /dev/input/event0 (OSMC Remote Controller USB Keyboard Mouse)
Feb 19 06:28:39 RPi4 systemd[1]: sys-devices-virtual-misc-rfkill.device: Failed to enqueue SYSTEMD_WANTS= job, ignoring: Unit systemd-rfkill.socket not found.
Feb 19 06:28:47 RPi4 systemd[1]: kodi.service: Deactivated successfully.
Feb 19 06:28:47 RPi4 systemd[1]: Stopped kodi.service.
Feb 19 06:28:47 RPi4 systemd[1]: kodi.service: Consumed 22.030s CPU time.
Feb 19 06:28:47 RPi4 bash[1466]: Starting SteamLink
Feb 19 06:28:47 RPi4 kernel: input: Valve virtual keyboard as /devices/virtual/input/input6
Feb 19 06:28:47 RPi4 (udev-worker)[1530]: event6: Process '/usr/bin/evrepeat -d 500 /dev/input/event6' failed with exit code 1.
Feb 19 06:28:47 RPi4 systemd-logind[380]: Watching system buttons on /dev/input/event6 (Valve virtual keyboard)
Feb 19 06:28:48 RPi4 bash[1466]: QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/storage/.kodi/userdata/addon_data/plugin.program.steamlink/steamlink/.tmp/runtime-root'
Feb 19 06:28:48 RPi4 bash[1466]: INFO: Warning: Couldn't set IPV6_ADD_MEMBERSHIP on discovery network socket
Feb 19 06:28:48 RPi4 bash[1466]: "Not supported"
Feb 19 06:28:48 RPi4 kernel: bcm2835-i2s fe203000.i2s: I2S SYNC error!
Feb 19 06:28:49 RPi4 kernel: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000014
Feb 19 06:28:49 RPi4 kernel: Mem abort info:
Feb 19 06:28:49 RPi4 kernel: ESR = 0x0000000096000007
Feb 19 06:28:49 RPi4 kernel: EC = 0x25: DABT (current EL), IL = 32 bits
Feb 19 06:28:49 RPi4 kernel: SET = 0, FnV = 0
Feb 19 06:28:49 RPi4 kernel: EA = 0, S1PTW = 0
Feb 19 06:28:49 RPi4 kernel: FSC = 0x07: level 3 translation fault
Feb 19 06:28:49 RPi4 kernel: Data abort info:
Feb 19 06:28:49 RPi4 kernel: ISV = 0, ISS = 0x00000007
Feb 19 06:28:49 RPi4 kernel: CM = 0, WnR = 0
Feb 19 06:28:49 RPi4 kernel: user pgtable: 4k pages, 39-bit VAs, pgdp=000000005f3ae000
Feb 19 06:28:49 RPi4 kernel: [0000000000000014] pgd=080000005f349003, p4d=080000005f349003, pud=080000005f349003, pmd=080000005f34c003, pte=0000000000000000
Feb 19 06:28:49 RPi4 kernel: Internal error: Oops: 0000000096000007 [#1] SMP
Feb 19 06:28:49 RPi4 kernel: Modules linked in: xt_conntrack xt_MASQUERADE nf_conntrack_netlink nfnetlink iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xt_addrtype iptable_filter ip_tables x_tables br_netfilter bridge stp
Feb 19 06:28:49 RPi4 kernel: CPU: 0 PID: 1529 Comm: shell Tainted: G C 6.1.74 #1
Feb 19 06:28:49 RPi4 kernel: Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
Feb 19 06:28:49 RPi4 kernel: pstate: 00000005 (nzcv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
Feb 19 06:28:49 RPi4 kernel: pc : vc4_plane_atomic_async_update+0x1b8/0x270
Feb 19 06:28:49 RPi4 kernel: lr : vc4_plane_atomic_async_update+0x4c/0x270
Feb 19 06:28:49 RPi4 kernel: sp : ffffffc00a59b880
Feb 19 06:28:49 RPi4 kernel: x29: ffffffc00a59b880 x28: ffffff806000d000 x27: ffffff804a042000
Feb 19 06:28:49 RPi4 kernel: x26: 000000000000fff8 x25: 000000000000fffd x24: ffffff804a042000
Feb 19 06:28:49 RPi4 kernel: x23: ffffff804a042400 x22: ffffff805ec5fb00 x21: 000000000000001a
Feb 19 06:28:49 RPi4 kernel: x20: ffffff8040f2dc80 x19: ffffff806000d000 x18: 0000000000000000
Feb 19 06:28:49 RPi4 kernel: x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000030
Feb 19 06:28:49 RPi4 kernel: x14: 3830317830323931 x13: 000000000ee3e000 x12: 0000000001db8c00
Feb 19 06:28:49 RPi4 kernel: x11: 0000000000024414 x10: 0000000000000804 x9 : ffffffc008780104
Feb 19 06:28:49 RPi4 kernel: x8 : ffffffc009c25000 x7 : ffffff805ec5fa00 x6 : ffffff805ec5fb00
Feb 19 06:28:49 RPi4 kernel: x5 : 000000000000202f x4 : 0000000000000000 x3 : 0000000000000010
Feb 19 06:28:49 RPi4 kernel: x2 : ffffff8063fd5780 x1 : 0000000000000001 x0 : ffffff806000cc00
Feb 19 06:28:49 RPi4 kernel: Call trace:
Feb 19 06:28:49 RPi4 kernel: vc4_plane_atomic_async_update+0x1b8/0x270
Feb 19 06:28:49 RPi4 kernel: drm_atomic_helper_async_commit+0x70/0x150
Feb 19 06:28:49 RPi4 kernel: drm_atomic_helper_commit+0x114/0x19c
Feb 19 06:28:49 RPi4 kernel: drm_atomic_commit+0xb0/0xf0
Feb 19 06:28:49 RPi4 kernel: drm_atomic_helper_update_plane+0x140/0x180
Feb 19 06:28:49 RPi4 kernel: __setplane_atomic+0xfc/0x140
Feb 19 06:28:49 RPi4 kernel: drm_mode_cursor_universal+0x11c/0x24c
Feb 19 06:28:49 RPi4 kernel: drm_mode_cursor_common+0x100/0x220
Feb 19 06:28:49 RPi4 kernel: drm_mode_cursor_ioctl+0x50/0x7c
Feb 19 06:28:49 RPi4 kernel: drm_ioctl_kernel+0xcc/0x180
Feb 19 06:28:49 RPi4 kernel: drm_ioctl+0x1fc/0x430
Feb 19 06:28:49 RPi4 kernel: drm_compat_ioctl+0xec/0x100
Feb 19 06:28:49 RPi4 kernel: __arm64_compat_sys_ioctl+0xd8/0x14c
Feb 19 06:28:49 RPi4 kernel: invoke_syscall+0x50/0x120
Feb 19 06:28:49 RPi4 kernel: el0_svc_common.constprop.0+0x4c/0xf4
Feb 19 06:28:49 RPi4 kernel: do_el0_svc_compat+0x24/0x4c
Feb 19 06:28:49 RPi4 kernel: el0_svc_compat+0x2c/0x8c
Feb 19 06:28:49 RPi4 kernel: el0t_32_sync_handler+0x98/0x140
Feb 19 06:28:49 RPi4 kernel: el0t_32_sync+0x190/0x194
Feb 19 06:28:49 RPi4 kernel: Code: 39044403 394f3263 390f3003 f9405263 (b8617863)
Feb 19 06:28:49 RPi4 kernel: ---[ end trace 0000000000000000 ]---