If it is, i'd just replace it and not worry about using the "stop leak". I am mostly against stop leak when a normal repair can be done without too much trouble.
I try to stay away with any sort of block sealer because they have the potential to cause problems. Not saying that they will cause problems, but I'd rather make a repair without having to add something to my cooling system that may or may not help.
If the freeze plug doesn't require hours of labor to replace, why not just replace it instead?