I've been doing these ab workouts where you lie on the ground, with your legs straight out and your arms straight over your head. And you just lift your legs 3 inches, and hold 'em there. That's it. Sounds easy but after a minute or two it gets tough to hold onto. Would that work for your back? It engages the lower abs without having to bend your back, at least in my experience.
Sucks that your workout is impacted... I hurt a wrist in college - playing flag football and acting like a damn fool - and I couldn't do pushups or put any real tension on that wrist for a whole year.