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Well, I rode 650b with 2.1 on my Wolverine during Gravelaide yesterday, and I had a couple of moments where the cranks scraped the rocks below. So not a suggested combo.
pascalo-
According to this - http://www.bikecalc.com/wheel_size_math - these look like reasonable options for matching 700c wheel sizes
wheel tyre size rim diam. (mm) tyre size (mm) total wheel diam. (mm) 26inch mtb 3.00 inch 559 76.20 711.40 650b/27.5 2.50 inch 584 63.50 711.00 700c/29er 45 mm 622 45.00 712.00 26inch mtb 2.25 inch 559 57.15 673.30 650b/27.5 1.75 inch 584 44.45 672.90 700c/29er 25 mm 622 25.00 672.00 Of course tyre pressure is a significant factor and you can't necessarily assume that you can just add the tyre "size" to the rim diameter to find the wheel diameter.
If you're hitting the cranks on 650b x 2.1" it seems like the same thing would have happened on any 700c tyre up to say 35mm or so. But I wasn't there so I don't know. Maybe shorter cranks are the solution? Or even bigger tyres?
So @5awyr if you find a frame that takes both 622 x 25mm and 559 x 2.25" or a frame that takes both 622 x 45mm and 559 x 3.0" you might be alright (caveat: can a normal MTB rim take a 3" 26"+ tyre? I don't know). Maybe have a look at BB heights on the geometry charts for a few bikes (e.g. Soma Wolverine, Surly Straggler, Genesis Vagabond, Salsa Fargo/Marrakesh/Vaya etc.) and see if any of them look about right. I think you'd have to go for some sort of monstour bike and just be content with being slow on road rides.
frankenbike
I have seen people use 26" wheels on the Soma Wolverine