I was discussing this with a friend today, I'm waiting for it to get to about 15 degrees and nice and sunny
TBH thoughif you rugged extremely well after theres no reason why you can't bath in this sort of weather (as long as its not below freezing.) I personally wouldn't but I know a few people that do
I had to bath mine last week as we had been on a pleasure ride the day before and he had got extremely sweaty. Was worried if i left him unwashed he would get sore. I gave him a bath and then rugged him up well with Thermatex type rug as the bottom layer, covered by a lightweight turnout and a medium stable rug. I left him in for the day, went back after about 3 hours and removed wet rugs and replaced with cooler fleece covered with stable rugs, removed these about 3 hours later as he was just about dry and put on normal stable attire for the night. Then spent the next few days with assorted rugs drapped around the garage trying to dry them out. Good thing i have a lot of rugs!!!
So yes you can bath at this time of year but it is a lot of extra work and you need a lot of rugs!