I almost wonder if at some point they considered having Luke be physically alive to some extent. Like, maybe Luke could appear physically, as if he achieved an even higher enlightment than the other Jedi. But that wouldn’t explain why they would have to conceal his hand… Very weird. I don’t see why, under any circumstance, they would want to hide his right hand, unless they briefly played with the idea of him having some physical form and they weren’t sure how to depict his right hand.
It was a question they didn’t want to answer or change the continuity of, they could have given him a fresh new hand or given him his mechanical; people asked this same question about Anakin in ROTJ and it was left unanswered think they’re simply doing the same with Luke.