As long as you backup the eMMC using "dd" first it shouldn't be too hard to restore it again if things go wrong. GXL devices boot the same from eMMC and SD card so once you've erased eMMC it's simple to test u-boot(s) from SD and when you've proven it works, you can write the boot stuff to eMMC. In the event you do screw up and need to erase eMMC the worst-case scenario is shorting pins on the eMMC chip to stop it being checked during boot, and then the SOC finds u-boot on SD or USB again.
IIRC there are some capabilities that OSMC runs from the secure world (BL32) to prevent them being copied (Dolby Vision?) so any mainline u-boot will lose those (not that upstream supports DV anyway). I can ask Sam for the boot FIPs (minus the BL32 bits I'd expect) and then it's a straightforward process to create a complete image. It's not something I'd have interest in adding to LE as the "box" approach with vendor u-boot works fine, but the ability to TFTP boot can be useful for testing.