I was wondering the same thing recently. If you run a 38-40mm tire on a 650B rim, I think you want a fender designed for that, which would be a 48-50mm 650B fender such as those that VO sells.

I think many 650B conversions cut it pretty close on brake reach so one intended for 27" would be pushing the envelope. It's nearly impossible to say, though without putting a wheel in there and mounting a brake.

For the reach you have to have, I'd rather have the additional efficiency (strength) of dual pivot, extra long reach sidepulls (like Tektro R559s) as opposed to a centerpul. The lower the brake pad sits in the slot, the less effective it is.

I do not think 650B wheels with 38-40mm tires will look bad so long as there are fenders. Quite the contrary, I think it'll look really cool.

There are lots of pics of taller frames converted to 650B in the 650B threads that are within this forum. Nlerner has done a few really nice looking jobs.