It is true that a bird of this size would suffer ill effects if left in a sling. However it could be used for therapy. Years ago I was involved with the rehab of an Owl. This sucker was huge. I think it was a Great Horned Owl. This gal had been hit by a car and suffered spinal dislocation. Though risky, surgery was performed and "Tigress" recovered well. So it was up to vet students and vet techs to rehab the bird. Back then not much was really known about this type of surgery on this type of bird. She was to be a learning experience for the students, and it wasn't known if she would be flight worthy. Though she was temper worthy! She was spitting mad every time we approached her and we had to be mindful of talons, beaks, wings and flying fecal matter.
At first she was unable to use her legs, so we fashioned a sling for her. We had to wrap her wings to place her in the device so she wouldn't flap herself to a panic. We gave her an hour or so a day to allow her to move her legs. She really didn't want to do anything. So somebody rigged up one of those baby things-I can't think of what it's called-you put the baby in it and it can scoot about-and we placed the owl in it. When Tigress figured out how to use her legs to move around she did so. This became her daily physical therapy. With lots of TLC that she absolutely hated, she eventually did recover enough to be placed in a zoo. She never regained flight but she can pounce on her food from a tree branch.
Now days it is considered a waste of money if you can't rehab a wild animal and return it to the wild. But Tigress begs to differ. She survived, we learned many lessons from her, and she still teaches today. At the zoo, she is the ambassador for her kind as people from all over stare at her in wonder.