Does anyone have an idea of a smaller overnight style bag that fits with a short rail backrest? (While riding 2-up) Most I'm seeing works with taller backrests and luggage rack.

 

Saddlemen, Nelson Rigg, Kuryakn, etc... all make some nice one that can fit a smaller sissy bar, you'll just have to look around. Try looking at Dennis Kirk or J&P Cycles for some ideas too. However, all of them will require a luggage rack while riding 2-up. Unless you go with a roll bag and attache to your handlebars.