The only decompile projects I've seen are the ones you've mentioned, and none of them have been Sega games. I would think that if anyone had seriously tried they would have done so first on Sonic 1, as a proof of concept principally but to spearhead the movement on Genesis games.
Coincidentally that doesn't mean it couldn't be tried on the SNES version, which seems to be where the majority of the effort is being placed.