So I've now tried importing completely custom geometry into Sonic Generations (as in made from scratch in Blender), and it worked, collision and all.
Except for the damn textures/materials.
I had heard it wasn't possible to import geometry from Blender to Sonic Generations, and that 3DS Max or Cinema4D was required...but that info was really old and involved more primitive importing methods, and I actually DO have perfectly working geometry taken from Blender. It's just the textures and materials.
I'm so frustratingly close to having this process mostly figured out and streamlined. While I do intend to learn 3DS Max in the future, I don't want to have to learn it just for Generations right now. I just want to be able to make mods relatively quickly in Blender since I know Blender, and I have the process mostly figured out already.
Blegh. I'll get back to it tomorrow. I've been up since before 7 AM and it's 2 AM the next morning now. Too tired.