Yes, I've done it with a disc brake equipped bike some years back. I wanted a disc brake hybrid back before they started putting disc brakes on hybrids. So I measured up stuff and found that a 700c rim laced to a MTB disc hub and set up with a 25mm tire would still have lots of room for a fender on a mountain bike. And that was on a suspension fork with the bridge between the sliders. On a rigid disc compatible suspension corrected fork there would be lots of room for anything you could think of since there's no bridge.
Just to reinforce, the REAL reason the forks are longer is that suspension thing that vredstein mentioned. Some of the really old bikes from the days of all rigid forks would be a tight fit for a 700c. Or maybe not..... If you measure the axle to knobs radius of a 2.1 inch off road tire and the axle to tread face radius of a 700c with a 25mm tire the 700c is actually less than 1/2 an inch bigger. Given the need for mud clearance on the MTB forks it is actually likely that even the old pre-suspension corrected designs would work. Braking aside of course.