OK, to close the loop on this (and maybe risk more joke photos...
), I came across the following today and have ordered some. The key info on the order page that helped by making the resulting fluid all-weather but remaining non-poisonous (as compared to the standard "blue stuff"'s skull-and-crossbones warning):
"Freeze prevention
303 Tablets may also be added to "anti-freeze" type premixes to provide the strong cleaning power most premixes lack. If you wish to add the tablet directly to water, you can add some isopropyl rubbing alcohol to the reservoir to prevent from freezing. To 1 quart of cleaner, add 8 fl oz to protect to 23 degrees F, or add 12 fl oz to protect to 7 degrees F."
From
http://www.sportscarcare.com/scc/product.asp?ItemID=303-was .