Concerning lenses, I am thinking (guessing?) that the Bausch & Lomb anastigmat on my poco might well cover 5x12. It seems to overcover 8x10 substantially. Look out for inexpensive decrepit pocos with tele anastigmats! Incidentally the FL of mine is ~11". It converts as well, by removing the front element... but offhand I don't know what that does to the coverage.
Maybe I can try to measure the coverage if there is interest.
Mike: I was banging my head against the wall trying to find lenses to cover 5x12 and this is precisely why I opted for 4x10... I don't have Jim Galli's stash of lenses! The next biggest reason was the cost and availability of film. Turns out I later got a huge supply of 5" roll film, but nevertheless, I think it is undeniable that there are many more options for lenses and film for 4x10. If you need movements I think you will find 5x12 very restricted indeed. And honestly with 4x10 I find that I am using way more shift than I thought I would. I mean, inches of shift.... the aspect ratio seems to make me notice perspective convergence much more strongly.... simply because lines along the long axis are never too far from the frame edges. I can show you examples of what happened when I shot the 4x10 with inadequate coverage whilst trying to tackle this problem.