Probably yes, but it wouldn't be upstreamable into fenster. A Wayland impl would at least double the size. I'm vaguely looking into it anyway. Feel free to file an issue

API compatibility with fenster would surely be the way to go here. It seems like it has the capacity to grant access to a huge number of platforms, but I'd hate to see it turn into a behemoth of ifdefs for the one codebase to do it all.

Independent implementation of the API would mean you could just release stuff and say "runs on anything that implements the fenster API" and then watch as a RISC OS version turns up somewhere down the track.