Fabricate a cover for the lock...plastic bag will work...could build a more elaborate cover out of wood metal etc....depends on your free time....other things
We got one at work that freezes on our gate...I am usually the earliest to work and I have a propane torch I keep just for that purpose...to heat it up.
Ive tried the plastic bag thing at work but people who leave later than I do wont put it on.....bastards.....so I torch that lock when its frozen....Ive also been known to take a large ball pein hammer to it also.....I just lay the lock on my bosses desk and tell him to get another....of course we dont use master locks....at least I dont think its a master lock as it is brass and a 4 number combination...