I can't imagine why it would not be legal when done during pre-fish days. Personally I would say it will likely be a waste of time.
A friend of mine was pre-fishing a national tournament a few years ago in Florida. He caught a 4 pound black crappie monster. He wanted to mount it really bad but felt if he released it, he might catch it in the tournament. I guess it was possible but you know the rest of the story--it hasn't been seen since by him.