How do the Batman Cuffs work in Batman Fluxx?
The FAQ already answers part of your question:


  Q: How exactly do the Batman Cuffs work?
  
  A: Yes, the wording on that Keeper is a bit vague. The intention is
  that you can hide one Creeper under the card on your turn. You can
  reveal it at any time, but the hiding can only be done on your turn.
  We will be updating the wording on subsequent print runs to make that
  clear.


Remains the question if you can hide someone else's creeper. 

We can'be sure what the intention is on this one. But based on several Fluxx set's I own, I assume you can't. There are a few cards that work on other player's creepers and these are spelled out. So it is not likely that you can use these for creepers you don't control.