I reheated my ribs while I was smoking some chicken at about 325. I put the ribs (in foil pan) just above the chicken and would stick my therma-pen into the foil to check the rib temps. They don't have to be "HOT", but just nice and warm.
PS. While doing chicken I'll start on smoke or 225 for the first hour and then crank it up to 325 to crisp up the skin a little.
Keep in mind that I'm cooking on a modified Traeger 075. So I can adajust the temps on the fly. I put the ribs in with the chicken (smoke/225) so as to not "shock" them going from the frig right into the cooker. I found that it took me about 2 hours to get the ribs nice and reheated.
As far as holding in a steam pan, you run the rick of "steaming them". I'd keep them in foil and keep the steam table at 140-160 if possible. Or what ever your health dept. demands. Check with them before you follow my advice...
Roaring Fork BBQ :-D
Modified Traeger 075