Well after all these years I have yet to find anything that keeps my bait from freezing at -30 and -40. You think you Californians have problems!
Don't worry about your sardines freezing. I have some out in cat, mink and fisher sets right now and believe me they are frozen. They still work.....sure the scent is not as strong but it still works.
Not sure what you are trapping, but a lure scent on a tree branch up above your set and an attractor, such as a foil pie plate or duck wing hanging from a branch and blowing in the breeze, can help when the cold is keeping the scent down.