There is absolutely no proper way to ask for cash gifts through a registry. In my opinion it is wrong. I've seen treasure chests on wedding Web sites that link to paypal accounts, I've heard of people not registering at retail stores because they thought that would encourage cash or checks at the wedding.
Look: You're going to get gifts that your guests want to give you. There's nothing you can do about it. But please do not register for cash. I know there are new sites that let you do that but it's tacky and it implies that you're greedy and in it for the money.
So just don't do it.