I'm open to the possibility of having to do that. I'd prefer to avoid it, just because I find it it easier to inflate a tire on the ground and then install it, rather than the other way around, but that's not a deal breaker.
I do have to be able to run full fenders though. Aside from the vintage-coolness factor, this bike is going to be a daily commuter for me. I really can't show up for work sporting a skunk stripe.

According to Sheldon Brown, 650B would give me 584mm diameter, rather than the 622mm of 700c, or even the 630mm it was originally equipped with.
I get the feeling I'm overlooking something really simple though, because the original set up offered more space over the 700c, but the 700c is physically smaller and doesn't fit? My brain hurts.

I'm not necessarily committed to any particular size, I haven't yet bought wheels or fenders, but I'd prefer to "upgrade" over the original 27"; just for the extra options it gives me for tubes and tires if nothing else.