I think pretty much any rear derailleur would do the trick once you locked it in place via the limit screws -- the tooth difference for most triples is 20-23 teeth, well within the capacity of most "road" derailleurs.

EDIT: I would definitely crunch some numbers on Sheldon's gear calculator or www.gear-calculator.com before ordering any parts. Some sort of faux half-step would be nice for breaking up the gaps between AW ratios...