That would be cool, but we need to think about the people who still use software mode. Since software mode uses the raycasting something-or-other engine, the game is essentially 2D and thus would not support 3D anything. It would be a cool addition to OGL mode though, which renders everything in actual 3D.
Like a441 said, you can prerender models as sprites and put them in SRB2 like that. Although I can't get this image out of my head of every badnik in the game looking like the current Knuckles sprite... *beaned*