I have a spare un72 bb which I will use with Phil rings for my wife's R20 rebuild, but any other R20 after that I think the rethreading is the go, I've got a LBS who is quite willing to help.
I've mentioned in previous posts that a local frame builder here is willing to make me a frame from steel or aluminium to the R20 design, but with a 68mm bb shell with 24tpi. He could also make the rear end for a 9 spd hub using a triple crank upfront and even lenghtening the downtube a tad for more cockpit room. Braze-ons for bidons and for using V-brakes, cantilever or even discs. It's very tempting to do it, just wish he would give me a price first......Oh yeah, he works with carbon fibre too.