I've been touring on a 29+ bike with 3" tires for 3 years now and love it. It rolls fine for 100kms+ pavement days, loves dirt roads and rocks techy singletrack very well due to the large diameter wheel. I can't see myself going back to "skinny" tires for the kind of touring I do.
I haven't tried B+ tires/wheels and probably won't unless I get a FS bike to tour on. However, I don't see any reason not to stuff a set into your KM if you get a BB height you are happy with and there is lots of clearance in the rear for it.
My buddy rode a B+ Rocky Mountain Sherpa on our last tour and he liked it a lot.