It doesn't matter what "the law" or the Army Regulation was - you'll have to look down at least to brigade and probably to regimental level to find out what was really going on. Which is why it was not uncommon to find blacks being used in a supporting role - teamsters, cooks, servants and the like - in spite of the fact that it was against the law to do so. But combat soldiers? Not likely.
Well, gg grandfather's slave carried a rifle and rode with the 51st Alabama Partisan Rangers (Company B). He was shot at (and shot back) near Nashville, and rescued another man in gg grandfather's platoon whose horse was shot out from under him.
I don't know how much more combat soldier you can get than that. I have no idea if he appears on the muster roll or not, since I'm not sure of his last name.