I've done a lot of hiking and walking, and a fair bit of that has been on single-track trails. I will say that on those trails, probably 98% or more of the trail, you could ride with any bike, provided it didn't fall apart. So I can see an off-road recumbent being useful on a lot of trails, although it presumably wouldn't be able to do some things a regular mountain bike could. It seems to me that the ability to get a foot on the ground in a hurry would be handy but would be especially awkward with the recumbents. The foot-forward designs look sort of goofy but might actually work better than a true recumbent on off-road.