Back in the 60's, a lot of British handbuilt frames didn't have derailleur hangers. Instead, there was a claw rather like the integrated hanger in the pic, which the regular derailleur screwed into, and which clamped into the dropout. I have even had Campagnolo Gran Sport derailleurs with this arrangement, on old Mercians and Claud Butlers. Might work in your situation. I probably have one on a crappy derailleur that I can send you. PM me, and I'll take a look.

I presume you want the quickness of track geometry, with gears? I always had that idea when I was a kid. Never tried it though.