If they're following USEF rules then you can't use hoof boots, even if they don't go above the coronet band.
From the USEF Dressage Attire & Equipment manual
: "Martingales, bit guards, any kind of gadgets (such as bearing, side, running, balancing reins, nasal strips, tongue tied down, etc.), any kind of boots (including “easy-boots”) or bandages (including tail bandages) and any form of blinkers, earmuffs or plugs, nose covers, seat covers, hoods are, under penalty of elimination, strictly forbidden."
I'd be inclined to skip the show, but if you really want to go you could either put shoes on just for this cycle, or ask your farrier about something like glue on shoes?