Doesn't just about every convertible frame work on that basis?If it's designed as convertible frame via dropouts etc then that's ok, they just re-position the axle. I meant someone re-speccing a 'new!' 650B bike based on simply those changes to a fixed-dropout 26".
On the 2.4" tyres on 26" rims point - I used to use 2.4 rubber queens on my 26" rigid SS. They were roughly 650B OD. TBH I'd prefer that tyre on a wider rim to a 650B x 2.1". Volume, PSi and contact area counts for a lot. Surly's 29+ is a similar thing.
The old system of measuring OD and altering rim dia to account for tyre volume was a good one, a standards nightmare that would produce a hundred threads here but the 650B+ tyre that I'd like to see is just a 29" OD based on that idea. 650Bx3.3" or so.
2.4" on a 26" is like 26+, or 650B. Volume of tyre can have the same or greater effect than simply rim diameter. Give me a fat 650B rim-tyre over a skinny 29er wheel.
2 ways to get the same result - less easy to hang a marketing tag on. And chucking in a few more variables would really confuse the "I want to see test data!" lot ) Don't think, feel.