Have recently been messing with 69er concept and swapping wheels between Road rat and SS 26er.
Big wheel in the back won't be a problem if you have clearance – why would it?
Raises the BB a bit but you'll cope if running such a skinny tyre.
As an aside, the 69er thing works well. On one carbon 26er forks and a 2.1 29er tyre on my road rat wheel was quite a bit better than same fork and a 2.1 26" wheel/tyre. 440 a-c rigid fork with big wheel and tyre sitting a smidge lower than unsagged 100mm maguras I usually run.
Felt stable, sure footed and worked in mud no problem but 26" wheel and a suspension fork still makes for a nicer ride on the whole.
My mates who tried it also liked the way it handled.
I should say the bike frame it's fitted to has a very steep head angle, high BB and it's also a compact frame purpose built for my 5'6" stature. I did not have toe overlap but clearance was much reduced to my size 7 tootsies.
May consider getting a new wheel and fork to suit next winter – the open pro rim being a bit narrow for the tyre I used, a 2.1 Schwalbe Little Albert.
Jumping onto a 140mm full susser afterwards reminded me how things have progressed and how much quicker down hill we are since the days everyone rode rigid by necessity though.