There are numerous tricks you can play with cyclocross brakes (interrupters) used as stand alone road ratio levers on all sorts of bars. One thing I am not sure of though is whether or not they will provide the proper clearance when installed inboard of grip twist shifters. They work just as well as normal levers except the mechanics is to push the housing rather than pull the cable. Works the same as the differential is all that matters.

They can be bought cheap or for as little as $50 a set you can get TRP carbon fiber levers. TRP, Tektro (same company) and some others make these levers in 3 band clamp sizes. 31.8mm for the center of OS bars, 26.0mm for the center of standard road bars and 24mm for the outer section of road mars or aero bars. This latter size will probably clamp on MTB bars without problem.