Having a Github repo with the needed dkms bits sounds good, go ahead!
You can include all that’s needed and maybe just put a script to re-sync things.
The SHIFTFS_MAGIC bit could probably be passed as a define directly to the compiler to avoid needing any code change at all.
As for how shiftfs work, we don’t shift to a particular user, shiftfs instead converts between user namespace ranges. So when you pass a mount with shift=true, the uid/gid you see in the container are now identical to what you would see outside of the container. It’s not tied to a single uid/gid.