Honestly, bathing in mid 50 F weather on a day with no wind in the afternoon sun won't bother most horses, especially if their coat isn't too long and you do it quick (gather a team of bathers if you need to!). Cover them and keep them moving in the sun.
I wash legs almost everyday in muddy, cold weather, but then again, I'm mean. ;)
Normally, I skip bathing for winter shows for my OWN comfort.