I don't have access to indoor washing so HE who could stand the cold better than others could see a bath clear into late Fall if the weather co-operated.
Until he hit his 20's and developed Equine Metabolic Syndrome, the horse in my avatar could handle a bath if it was 60 and the sun was shining.
Two more are good if it's "only in the 70's and the sun is shining.
The fourth horse, needs the temps to be mid-80's before he thinks that's not ice water coming out of the hose
As far as washing just the legs. If I have to wash legs in the winter, I carry enough hot water to the barn to wash with a cleanser (generally Dawn dish soap for just legs), then rinse. I towel dry them the best I can.
When it's too cold to bath, out comes the shop vac. Everybody loves the shop vac and all those little brush attachments that massage their faces - well not everybody - "Step Away From The Arab's Face" with the vacuum