IIRC, the original impetus for using 650B tires was so you could get fat tires on your 700C frame. Tires that were really, really wide -- like 32.

Oh, your bike can take 700Cx32s already.

So now, you're wondering how fat a 650B tire you can fit in there -- probably because one guy has been pushing 650B really hard for a decade or so. If you've been reading Heine's humble hymns, you'll note he really is pushing flexible tires -- sidewalls and treads. Of course, the normal way to get flexible tires is to thin down the sidewalls and treads, accepting reduced puncture resistance and tread life in the name of greater speed and comfort.

That's great on a brevet -- 750 miles and you're done. And your load is 10-15 pounds.

But you're talking about carrying a spare tire, leading me to believe your touring load is going to be 2-4 times greater. ( => More tread wear, shorter tire life.) Most loaded tourists average 10-12 mph on the road. ( => Speed isn't everything.) And though you'll probably hit some bad roads, if you're even considering staying with 700C tires I'm gathering you're going to be mostly on roads. ( => No need for a fire road-capable mount.) The load you're carrying on the bike will help damp out a lot of the road shock, since it'll force the tires to soak up more of the road shock, though not all.

So no, I don't see any sense in it. But go ahead and try 650B wheels and see how you like them, if that's what you want to do.