there are a few 650B "XC" style out (KHS has a nice round up) and a few more are coming this yr. This is the one I'm looking at... http://khsbicycles.com/bikes/2013-kh...sixfifty-3500/
My big problem with the 29er's is the steep head tubes and high stand over on a long frame.. each is a minor thing but all three add up
To be honest, I find that I like bikes with steep HTAs anyway. I might be in the minority—and I'm sure that it depends on the bike—but the bikes that I've tried that have more relaxed HTAs felt sluggish compared to what I ride now. Stand-over is something that I hear you about though.
I'm really not sure about 27.5 bikes. I tested a Jamis Dakar XCT and it didn't feel like a big enough difference from 26" to me. Again, I'm sure it depends on the bike, and I've read a really great review of the 3500. Boston doesn't have any serious KHS dealers, which is a bit of a problem.
I can't make the the MBAS, but thanks for the heads up.