while i have no vital help to offer, when i was running a two ring/one cog setup, i used an old road RD. but i had a jump of 12 teeth. and i would just get a front derailleur and an old suntour thumb shifter to shift between rings.

also, you can call your setup a "dingle." it's fun to say and people look at you funny when you say it, then they usually laugh (with, not at, you).