I emailed VO about this a while ago asking about 650b x 42's and fenders on a 51cm frame. Sounds like it would be tight. Here's the very helpful response I got:
I threw some 650B x 38 tires onto a 51 cm frame we were checking over today. Starting at the front, 38 mm tires fit fine in the fork and the inside width is 50mm right under the fork crown, so the snake skin's would fit fine.
In the rear, where the chain stay bridge is, the width is only 38 mm, but you'd need to move the fender out about 15 to 20 mm to get the fender line correct. So you might end up having to crimp the fender down there to get it to fit fine. Up at the seat stay bridge, it is 58 mm wide, so it would fit there all right..Tire wise, in the rear, it would fit fine.