Thank you to everyone who responded
I appreciate all your help and great ideas. Here's what I did:
I decided that my desire to bathe my horse was more about my level of comfort than theirs... Truth is, they are perfectly happy out in the pasture, all muddy. They are well fed and out in the fresh air
Soooo, after a good hard (but rather warm) rain... I put them in the barn... their stalls freshly bedded with shavings... and let them roll to their hearts content. Also gave them a nice flake of hay and some sweet grain. Next morning they'd rolled most of the mud off and were dry... then I gave them a good curry and grooming and they look pretty good...
I'll wait for the warm Spring weather for a good bath. Thanks again everyone!