I use glycerin, available from the chemist, to stop eyes freezing when trotting for grayling in the winter.
It does not affect mono, but I could not say if a fly line would be ok.
Another ploy might be silicon spray. We had to use it on Cassies Alpha because when you opened the door, the windows dropped an inch to clear the rubber seal. In the winter, the window would freeze to the seal. Spraying with silicon (a dash board cleaner) stopped it happening.
I know some people use silicon sprays on their fly lines to increase shooting distance, so I presume its safe to use.