Go for it! I sprained my ankle once and had to poultice it (yep used my horse's poultice) the night before a comp so I could get a boot on over the swelling. Luckily it was my right ankle so it didn't hurt too much getting up in the saddle.
It was a combined training so dressage in the morning then SJ in the afternoon, never took the boot off all day. When I finished SJ in the afternoon and had packed ther horse up etc, found I couldn't get my boot off for love nor money! Had to cut it off around the ankle. So just choose your footware carefully!
No pain no gain