I was thinking more along the lines of bow or muzzleloader season on private ground. Also, we have paint horses, so they shouldn't be mistaken for deer. Our horses wouldn't even pass for "pie bald" deer. I will grant you that riding a horse during shotgun season wouldn't be a good idea, private land or not.
Even if they did look like deer, isn't it a sad statement that a person can't ride one during shotgun deer season for fear of getting shot? You know the mantras, positively identify your target, never aim at anything you do not wish to shoot, etc. I wouldn't even go so far as to classify shooting someone on horseback as an accident. Borders more on negligence. Accident or negligent is a moot point if your dead though.