Hmmm. What about a Sturmey-Archer shift cable pulley? Clamp it down by the bottom bracket, run the cable around it, it'll reverse the direction of the cable so it'll be coming up from below the derailer. Will it work? I dunno, but it might be worth a shot.
Oh, wait. Upon closer reading, I see that you have considered this and couldn't find anything that would clamp your tubes. Well, you might be able to find away to come up with a way of clamping it. I can't really think of any other way to solve the problem.