I have a similar set up as the OP and I have an early 2000's XT derailleur on my cross bike. I had the frame made custom so I caused this problem for myself as I wanted the cables ran along the top tub to keep the dirt out. Anyway, while the XT still works, I've tried to use the clamp on problem solver pulley but the seat tube on my bike is ovalized at the bottom bracket junction and the clamp doesn't fit. I even bought a nice new derailleur to put on...still sitting in the box. If you can find an older XT like mine it is at least silver and would not clash as bad as the gray XTR. I don't look forward to having to replace my XT any time soon due to the problems you are having finding something new.