I've done two 650B conversions. One was a new Soma mixte frame built up for my wife with a 650B wheelset and extra long-reach Tektro R559 brakes. It wasn't so much a conversion, per se, but more of a build conceived from the start, around a frame that was desigend for either 700C or 650B wheels:
The other was a true conversion of a 1980s Japanese touring bike built around 27" wheels. This was more of a challenge because the frame had cantilever brakes, and the mounting bosses had to be removed and rebrazed to fit 650B wheels. You wouldn't want to do this on a frame that you want to keep original, but I made the decision to do this knowing that the frame didn't come from any particular provenance and that I would be powder coating it anyway:
siggurd-- you can mount a bell to the unused shifter boss!
woodrupjoe-- a frame made for 27" wheels presents an extra challenge because it may not be possible to source brakes with enough reach. Bikes built around 27" wheels require, on average, an additional 23mm of brake reach compared to 19mm when converting a bike designed around 700C wheels.