I've done plenty of backflips off of plenty of things (picnic tables, cars, diving boards, rocks and the ground, among others), but never off a wall. I'm 110% sure I can do it, I've just never tried. In theory it should be easier than a flip off the ground (on account of the fact that there's more height for the flip, and you technically only have to do 3/4 of a flip instead of the whole thing, since the flip starts relatively horizontally, more or less parallel to the floor), but I've just never tried.