Yeah, PG, so how about this idea...
15 gallon sanke keg with 10 gallons water, and 5 gallons of PG giving you a freezing point of around -10C.
Pop it in your deep freeze and get it down to about -5 C, then hook up a Sanke tap up to a pump, and recirculated the chilled antifreeze solution back into the 15 gallon sanke, if my old math is correct and I'm sure you'll point it out if it's wrong, starting with wort at 95C (having cooled a bit during your whirlpool) you have 100 degrees of thermal buffer or whatever that would be called in your keg and 3 times the volume, so to calculate the finish temp take the total of 20 gallons of liquid, 5 gallons of it is wort, so that means that you could get the wort down to 25% of the difference in temps, or 20C... (-5 + (100/(20/5))...
Is that right Dean?
JQ