You could also add a second gravity-feed tank and use Motortrol valves with the
soap function to energize them so the pump gets hot water only on
soap and cold on rinse and wax. The zero-pressure regulators aren't very reliable, but they do work.
I've wondered for a while now if it would work to use something like a Dosatron to inject the wax into the city-pressure feed to the pump. I keep checking eBay for a cheap, used one to try it, but they always seem to go up over $200.