The reality is a bale elevator is the way to go - it's a lot easier on the muscles and saves a considerable amount of time.
If you were thinking of a pulley type system, you could hook something up that would pull a couple of bales at a time up the stairs (assuming it's regular stairs and not a straight up ladder stair (which would be quite a feat for you bringing the bales up manually)) on plywood cut to size. The pulley would connect to the loft wall and then attach to the plywood sled. To make this work, you need to have the stairs located to accommodate the system, a sturdy wall frame, a reinforced connection in the plywood (otherwise wood might crack from stress) and a way to keep the plywood from slipping off the stairs if there is no railing in place.
If you get that to work, I'd love to see pictures of it in action. Good luck with the project.