I don't think you'll ever get ride times at h/j shows like dressage. Maybe the block times people have mentioned.
But if shows didn't allow same day adds & said horses had to be entered by oh say 5 PM the night before for the next day classes, they could at least print off class counts & people could get ideas of when they may ride. Someimes I am in the 5th or 6th class on the schedule...but it might go at 10:30 AM or it could be 1 PM depending how many are in the classes before me. That type of information I would find very useful.