I'd say bite the bullet and replace the freeze plug.
I've used rad sealers before, with varying degrees of success. Some actually sealed up, some didn't and some even clogged up system components like the heater core, etc. I used to keep a can in the tool box when I went fourwheeling, just in case I developed a leak in the middle of the woods and had no other options.
Whatever doesn't get consumed in sealing will settle in the bottom of the cooling system.
The bottom of everything in the cooling system....they're best used to get you out of a jam somewhere when you can't do a proper fix. But the cooling system should be repaired and flushed at the first opportunity.
Personally? I'll never use one again unless I'm in the middle of nowhere and have no other choice to "get back".....