You need a derailleur that goes with the biggest chainrings (road) but you need one with the proper pull ratio for mountain shifters. I would say, "ask RANS" but they've just changed hands and getting to them is somewhat difficult. I recommend that you ask over at BentRiderOnline, in the technical section. There are some good mechanics over there who know exactly which derailleurs to use. Also, you may need a chainring guard, to help keep the chain from shifting off on the outside.