The thing with the hula hoop is news to me and definitely something to be aware of as I can see that happening. To me that makes it even more important to practice with them though as you KNOW you'll be asked to in the ring. It's much better if your horse gets used to it and trained for how to handle any "behavioral oddities" on the part of the hoop at home, where he's comfortable and you can work him through it on your own time without stress.
(I see this as being similar to riders who refuse to introduce their horses to a carriage because "he's too scared." Hello?! What are you going to do, spend the rest of his life avoiding things instead of building his confidence? In this case a driving horse can't afford to avoid scary sounds and sensations because accidents do happen and he'd better be prepared and know how to react.)
I'd start in-hand and get your horse used to walking through the hoop, backing into the hoop, pivoting in the hoop, etc., then have him drive through it and work up to doing the full thing in the cart. He should be taught that it's no big deal if the hoop flips up, in fact I'd clicker train mine to associate that sudden flip with food so it doesn't bother them at all! I'm crazy that way though.
Leia