There is a belief that there is less oxygen to rust the base plate on the mud layer at the bottom of a canal, maybe so, maybe not.
Steel rusts less in winter due to the lower water temperature, fact.
How are you going to clean and paint the base plate? If its a dry dock the plate is partially hidden over the stocks anyway and maybe only 2' off the ground.
If its a drag out you will not get under it at all.
Paranoia about boats rusting through and sinking is normal but unfounded, very few do sink. Many are now 50+ years old and were built with thinner steel than 'yours' , some are only 3mm thick from new!
If you get iced in this winter and have to move you will destroy the blacking/epoxy anyway so leave it till spring.
If you are still worried about pitting, rock the boat over by loading up one side with drinkers, get a 12v bulb on a stick, put it in the water before you turn it on, and have a look at the top 6" of the sides, that's the bit which will be the worst.