"no more trying to keep the dog from 'previewing' the presents. No more ornaments as playtoys on the floor."
LOL. I don't think anyone's going to be sticking the presents to the ceiling, and the law of gravity will not be suspended along with the tree, so I'm curious ... what do you mean here?
OK, If I were really going to do this I'd probably either keep the presents on a nearby table, or I'd 'hide' them and put a note on the tree with each person's name and a clue as to where the gift was located. Then there could be an easy scavenger hunt associated with present opening.
Gravity wouldn't be suspended, but most of the time the problem with decorations finding the way off the tree isn't gravity. It's the critter wandering by unable to resist the plethora of 'toys' hang right in front of her face! Seriously, I can't see doing the upside down hanging tree but for those who aren't married to tradition, it could be fun (and not necessarily IMHO sacrilegious) YMMV:)