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I think you might struggle a bit to make that work unless the frame has a really high BB.
Smaller wheels lower the bike, and the reason why you can switch between 700 with skinnies and 650b with large tyres is because the diameter ends up nearly the same. That however is not the case for 26".
Of course if the frame was specced to run well on let's say 650b skinnies, maybe it could work with 26" also.
In any case, make sure to keep in mind that the BB will probably be lower for the off-roady wheels, and that will increase your pedals striking ruts and boulders etc.
pascalo-
Isn't the Rawland Ravn design for this. Fat 26"/ mid 650b/ skinny 700c. You'll not be getting hold of one of them in a hurry, but maybe have a look at the geo to see how it works?
https://www.bikeradar.com/mtb/news/article/rawland-cycles-launches-ulv-and-ravn-48176/
Polygon
Question for the gravel crew. I currently have a road bike and a mountain bike, both of which are beyond economical repair and neither of which are in a rideable state. Therefore looking for something new and, inspired by a thread in current projects, I'm thinking about trying to combine the two as I don't really have the space or time to keep a full mtb much as I'd like to, and therefore looking at CX/gravel/adventure/whatever the new word is bikes.
One of the key things I want to check is the feasibility of using 26" wheels with wide offroad tyres on these frames/forks for when I want to ride what I would have normally taken my mtb out on, and keep the road wheels for everything else. Reason being I've got a nice pair of 26er Hope/Mavics that I can't bear to sell but also don't want to leave unused. Obviously I'd have to do some axle conversion but that's easy on Hopes so my main concerns are clearance and how wide I'll be able to get away with, whether I'm going to ruin the geometry, and also freehub compatibility (its currently got an mtb spaced 9 speed cassette and I vaguely remember hearing an 11 speed needs a wider freehub...?).
(sorry this is so terribly written it was a bit stream of consciousness on my mobile)