Not quite sure I understand what you are after, but I'll try to help if I can! Could you try to give me a more detailed explanation?
Sounds like you could achieve what you want by just limiting the players Y movement! But like I said, maybe I'm not fully understanding what it is you are trying to achieve!
I'm going to be out for most of today, so probably won't be able to look at it until tomorrow.
Or perhaps, you could start another thread for this specific question, and someone else may be able to help you in the meantime!