The surgery may make the horse appear comfortable, but continuing to work a nerved horse while not fixing the mechanical issues helps to speed up the degradation of the bone.
That being said, correcting the angles (lowering the heel to utilize the shock absorbing structures of the foot instead of raising the heel to cause numbness) can help slow or halt the damage being done internally. Once you get the foot set up the way it should be, exercise wouldn't be harmful and actually helpful.
One reason the upright heels doesn't work is the tendon rubs over the nav. Bone is now tighter as the foot comes down, actually providing more crushing force on the bone (the nav. Bone is just a fulcrum point for the tendon that attatches to the coffin bone) and usually the tendon itself is more damaged than the bone initially. Also, the back of the foot acts a a shock absorber because it's made of cartilage. Lifting the foot with high heels takes this out of the picture and now the tip of the coffin bone is jamming downward, prying at connective laminae (creating a founder of sorts) and one more intresting thing about raising the heels...if the back of the foot is used properly, the blood that is pumped by the act of taking a step actually helps to diffuse shock as well, research indicates it's a "hydraulic cushion" so the act of moving actually makes the foot feel better than standing around. But if your horse has jacked up heels, he's not getting full benefit.
Sorry to make this a long post, but nerved or not (I don't recommend nerving) the heels should be allowed to work normally. Nothing can repair the damaged bone, but the pain can be alleviated by a proper heel first landing.