Do yourself a favor, try it on the wakeskate, until you get the feeling of being laid out, and pulling your feet ALL
the way back underneath you. Try to land with your feet out in front of you, as if you were in the sit and reach stretch position... Do this, because the board, although it doesn't weigh much, does slow your feet down, when you start trying them strapped in.
You don't have to be any good on a wakeskate to try a raley. It was harder for me to cut out (on the wakeskate), and then cut in really hard, like you need to, for a raley. Once I strapped my board on, I could cut even harder, which gave me more pop, and more time... Which is a huge help.
Trampoline question: I learned the raley at the end of last summer. I can pull them consistently, all day long. Just the other day, I tried one on a trampoline, and couldn't do it. No matter what I tried, it never once felt anything like it does when you're on the water. Also, it's pretty dangerous to lay out like that, and then try to pull yourself in on a trampoline... Chances are, you're going to pull yourself too much, and eat it off the side... I don't speak from experience
Let me know if you have any more questions. It's not a hard trick, but it takes a huge commitment to land one.