PG is in fact the main ingredient in the high-priced antifreezes sold as "animal afe" or "environmentally friendly". But since antifreeze (both the "safe" PG type and the regular stuff) has a bunch of additives, it would not be a good idea to use it in your humidor.
Straight PG is just fine though. It is also used as a preservative in foods for human consumption. You could, in theory anyway, drink a whole bottle and you'd be fine. Though I don't know how good it would taste.
-Lenny