Thanks for the responses.

I've got in the neighborhood of 200 or so hours in T-34's and C-12's from flight school, so I could get the extra 50 or so hours flying airplanes in my off time (correct me if I'm wrong, but ATP-Airplane requires 250 hours in airplanes). I'd essentially be looking at being a guy with 250 airplane hours, around 2500 tiltrotor hours, and a few other things like Aviation Safety Officer, CFI/CFII, NATOPS Instructor (like a Stan-I) and a few other military "I" quals.

I guess that would be the crux of it. I could get up to the minimums for ATP in airplanes, but then the bulk of my experience would be tiltrotors.