I've had long-haired dogs since 1998, cats since college, and have worked in two grooming shops and volunteered at two humane societies as a groomer. The owners of the shops had radically differing ideas on how to work with mats on cats/dogs, but perhaps this technique (modified only slightly from what the second owner had me use on cats) would work on rabbits' hocks/feet (I'm looking for that first "Spinner Friendly" bunny):
(1) Wedge the comb between the hock and the matt parallel to the hock so that the teeth of the comb are visible on both the near and far side of the matt. Use the scissors to cut the matt off, (the rabbit's skin will be protected from the scissors because the comb will prevent the blades from contacting his/her skin). Don't force the scissors closer to the rabbit; just cut off what the comb will permit you to.
(2) After the scissoring, remove the comb and use it perpendicularly to the normal angle (that is, use only the last one or two teeth) to separate whatever's left of the matt into smaller bits. If your fingernails are longer than mine, perhaps they'll be useful at this stage of the proceedings.
(3) After separating the matt into small bits, it may be worthwhile to clean the rabbit off with a washcloth or whatever you're comfortable using on that particular rabbit. Gently blow-dry the hocks--if the rabbit will permit--or towel-blot then -dry the hocks, following with a brush-out to make sure no proto-matts are left behind.