I think some of the idea is that couples can mask the fact that the gift being requested is actually cash. Cash is considered a crass gift, so if it's 'scuba lessons,' Aunt Mildred may not feel squicky about giving it. ANd, yes, that's do-able with a lot of other registries, too, and it's equally bold to register for expensive china with the intention of using it to get more cash.
I don't really see how these registries would work, anyway. I can't imagine most couples wait until they find out what 'the take' is and THEN make honeymoon plans. I know I couldn't just decide to upgrade my suite or to fly somewhere, etc. without some advance planning. I would think most people who want to take a big honeymoon trip would have budgeted for it beforehand. After all, what if nobody wants to donate?? Do you cancel the trip to Greece on your wedding day because you planned a trip you can't afford without donations??