I believe that the SDHost driver has now been fixed... Phil found the problem talking to the eMMC devices recently and a fix should have been uploaded to the latest firmware.
Adding OSMC to the Slice recovery system shouldn't be too difficult, it's actually more about where it'll be stored that needs to be sorted out really (Otherwise it's exactly the same as a normal NOOBS tar of the filesystems)
The main patches to the kernel are for the following:
1) CS4265 codec implementation (there are some modifications to the stadard one because we output the audio reference frequency to the device)
2) WS2812 driver
Then there are a couple of config.txt changes required to add the slice dtoverlay
For the Kodi stuff, the main things you'd need are just the changes to the main window code to load up and run the LED patterns on the various events.
Everything else is stuff that you shouldn't need (i.e. changes to work with the Slice website for updating the different build types, 'nightly', 'bloody' or 'stable' and a couple of minor tweaks for things like pasting debug log to pastebin)
How are you building OSMC now? Do you have an automated build system I can replicate?