Oh, the poor dear! She's very lucky you found her, SableCollie.
I've never had a bunny that was matted, but what I've done with dogs that have gotten badly matted is to cut the mats off to about a half inch to an inch from the skin. With dog hair, this actually make it easier to comb the mat out without pulling too hard. I can usually tease the mat apart from the cut edges with a fine-toothed comb--don't dig deep with it--just sort of comb the edges at an angle and it loosens up.
Maybe try it with a small patch on her belly and see if it works with rabbit hair?
Good luck with her--I hope she's feeling (and looking) better soon!