Propylene glycol is food grade and can be found in foods as an additive. You wouldn't necessarily want it to be in your beer, but if a little bit did get there, it wouldn't be dangerous. Ethylene glycol has no place in the brewery - at least in a situation where it could come in contact with beer. It is dangerous to ingest. The term "anitfreeze" implies ethylene glycol, whereas the term "glycol" implies propylene glycol, at least in the context of a homebrew forum.
I have tested and retested my chilling coils connections and I would be confident adding glycol to the reservoir to bring the temperature down. I suppose I could move my bottle and yeast collection for a few days during the time when I would want to crash cool the beer so that the freezer could get the coolant in the mid-20's.
My next question is how to get that damn lid to seal better. I have tried a couple of different kinds weatherstripping, but I can't get it to work. I guess that is better off in a new thread.