If you turn the pots upside-down, they can't fill with ice. Maybe cover them with a tarp, too. I can't imagine any sealant could be stronger than ice. I think a sealant is to protect the clay from absorbing so much water that ice crystals crack it from within the clay. It also helps during the growing season to prevent the water in the soil from evaporating through the porous clay walls so quickly. Are you able to use plastic pots for the plants while overwintering?

I agree that it's the water in the clay itself that does the greatest damage. You can store clay pots outdoors if you keep them dry. I have lots of very large terra cotta pots that I tip upside down when they're dry & overwinter on my growing benches under the cover of garbage bags wrapped tightly around them. For the pots with plants. I think I would be thinking about storing them where they can't freeze, or just slipping the plants out of them and into nursery cans or plastic pots for the winter so you can store the clay where it will remain dry.

Al