I have a 2x10 speed Tiagra 4600 700c bike that could probably fit up to a 700x45 tire. My hybrids 26 inch wheel with an 8 speed cassette fits fine on that bike, even shifts reasonably well through all 8 gears and I'm sure with a little DER tweaking it could improve (4600 10 speed brifters and DER have close enough pull ratios to the Shimano compatible MTN 8-9 speed stuff). My 26x2.1 Smart Sam tire is pushing it and rubs ever so slightly but it would work if I had to and anything under 2.1 would fine. I put it on only as a test and a spin around the block just to see if it would work. I never tried the 700c wheel fit on the my 26 hybrid, no need for me. Keep in mind that a 26 tire will end up sitting further behind the BB or lower in your fork, both are curved/arced areas and have more clearance at those positions possibly allowing a wider 26 tire to fit where an equal width 700c might not.

To stay on topic.. None of what I specifically have or tried on my stuff really helps anyone else other than the idea. In an "emergency" when parts are limited, it is good to know what options you have.