Ivory soap has been a favorite bait for channel cats in ponds. I've also seen folks using it in the river for blues but don't know how productive it is. I can't imagine it being illegal though.
I used to run with a commercial fisherman on the TN river. Ivory soap cut into 1 inch squares was all we used to bait trot lines with and we caught TONS of cats.
It is a favorite I believe of commercial fisherman. I know some of the use Ivory soap on trot line and also marshmallows. It is very productive. I seen of of the cathces.