I have found a solution for LibreELEC 9.2.0 (generic x68_64), but you have to apply patches and compile your own LibreELEC.
All files and patches mentioned below are in the attached Zip-file.
Copy the files linux-100-igmp.patch and linux-101-ip_sockglue.patch to http://LibreELEC.tv/packages/linux/patches/default/
Copy the files kodi-100.26-url.patch and kodi-100.27-ffmpeg.patch to http://LibreELEC.tv/packages/mediacenter/kodi/patches/
Then compile LibreELEC. => Enjoy 
This is what I have done:
I've written a python test script (SSMTest.py) to connect a socket to the source specific multicast group and close the socket after 2 seconds.
When you make a tcpdump while running the script, you should see something like the dump below with "allow" and "block" messages and the ssm address.
You do not need to have MagentaTV to test this.
With "tcpdump igmp -vv" you should see something like:
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
14:22:50.972890 IP (tos 0xc0, ttl 1, id 0, offset 0, flags [DF], proto IGMP (2), length 44, options (RA))
Kodi.fritz.box > igmp.mcast.net: igmp v3 report, 1 group record(s) [gaddr 232.0.10.146 allow { 87.141.215.251 }]
14:22:51.872890 IP (tos 0xc0, ttl 1, id 0, offset 0, flags [DF], proto IGMP (2), length 44, options (RA))
Kodi.fritz.box > igmp.mcast.net: igmp v3 report, 1 group record(s) [gaddr 232.0.10.146 allow { 87.141.215.251 }]
14:22:52.976230 IP (tos 0xc0, ttl 1, id 0, offset 0, flags [DF], proto IGMP (2), length 44, options (RA))
Kodi.fritz.box > igmp.mcast.net: igmp v3 report, 1 group record(s) [gaddr 232.0.10.146 block { 87.141.215.251 }]
14:22:53.336227 IP (tos 0xc0, ttl 1, id 0, offset 0, flags [DF], proto IGMP (2), length 44, options (RA))
Kodi.fritz.box > igmp.mcast.net: igmp v3 report, 1 group record(s) [gaddr 232.0.10.146 block { 87.141.215.251 }]
But with LibreELEC 9.2.0 (generic x68_64) I do not see the "allow" and "block" messages.
I searched the history of the linux file net/ipv4/igmp.c and found a fix in the newest version of the file that fixes at least this bug
So I applied this patch and compiled LibreELEC.
With this new version I was able to see the the "allow" and "block" messages, but I still could not see them whrn i tried the "PVR IPTV Simple Client".
So I applied some patches from the discussion in the kodi forum, which I already mentioned above.
After that I can see the the rtp streams with ssm addresses.
It now works with PVR IPTV Simple Client and TVHeadend 4.3 (not tested with 4.2).
With TVHeadend 4.3 you have to set Interface to e.g eth0 in the Web-Interface under: "Configuration->DVBInputs->Muxes"
