What’s your take on ad hoc modifications to footprints? In later versions of KiCad, reloading footprints is one by one, not for a whole library, but you can edit individual pads in case you want the pad to be different for this board only.
(I'm not necessarily asking for that feature because it could add complexity.)
I tried per-part revisions in earlier versions (so each individual footprint and symbol had a revision property) but felt it added too much complexity for not much gain. As it stands a Razen project has a list of dependencies (libname, revision) so each part from a library is loaded at that revision.
You could, in theory, branch a library to create board specific modifications to parts. But the issue there (also why I moved away from part specific revisions) is coming up with a good UI to manage the various part revisions/branches/etc.