Ive shot for up to a half day with temps in the teens. I put my developer and silver bottles in a cooler with hot water. You do have to pour the silver back into to the bottle to rewarm it in the cooler when it gets too cold to work. This has worked well for me when traveling around from location to location.
Just make sure the chemistry isnt too hot when you go to use it.
I will also heat the plexi baths before putting the chemistry in them with hot water. Also be careful doing this. If the water is too hot it can break the seams apart on the cold containers. I know because Ive done that.
As already suggested putting the plates in your pocket can help. It doesnt take long to heat a plate in an inside pocket of your coat. Of course this could be difficult if your shooting big plates. Unless you have a Capt Kangaroo coat