You could just buy a 650b frame and use that. The major difference will be BB height. If say you got an Aeris, in 140mm mode the BB height on a 26 wheel will be roughly the same as 650 wheel in 150mm mode (at full compression – within a few mm anyway – close enough that fork choice could erase the difference). So it would work fine in 140, although 150 would be sketchy but it would technically work. The tendency for long wheel bases on current bikes with long front ends means that the super short rear end doesn’t really work anyway as the balance of the bike is all wrong, so even though a 26 wheel could allow the rear to go shorter, you’re probably losing out rather than gaining if it did.
Just my thoughts as a lot of people do ask ‘can I run 26 wheels’ and this is what I tell them.