Yes, it can be "popped back" into proper position
Yes, a locking stifle can be popped back into place, but as I said in my earlier post--if the stifle is locked, she would not be able to position his foot flat on the ground. As long as the stifle is locked, with stifle & hock locked in an extended position and the fetlock flexed, it is anatomically impossible for the horse's foot to be placed flat on the ground. The only way that could be done is to unlock the stifle--and IF the stifle is locked in this instance, she has not described anything that suggests the stifle has been unlocked--if it were simply locked, once it was freed he would be able to walk (at least a step or two), up until it locked again. What she is describing does not sound at all like a simple case of locked stifle.
It's very dangerous for someone to describe ways to unlock a stifle in a case like this. Fortunately the owner hasn't tried any such thing. It could very well be that the horse has a fracture somewhere, or perhaps something (even the stifle, though it doesn't really sound like it's that) dislocated. If in a case like this the owner were to take the well intentioned suggestion that backing up can fix the problem and try to do it, it could cause untold harm (not to mention EXTREME pain) to the horse.
As far as stabilizing the horse for a trip to the vet--a sling may work, Besides a band under his belly I would be inclined to put a strap across his chest too, just to give him some extra front to back stability--something along the lines of the sort of sling they use if they have to air lift a horse out of somewhere. I'm not sure if that's a benefit in a trailer type sling or not?
I do hope you can get something figured out so you can get him to the vet soon. The poor fellow has to be very sore to be standing in one spot & just moving his front end around a bit. It really does make me think pelvic fracture--I had a Morgan mare that broke her pelvis one winter, and that's very much what she was like. I've never seen a horse so sore.