Yea I think pre-soaking a heavily soiled car with WW would be ok too...But seems like I get lots of "solids" on the rocker panels and such that would need to be blasted off before wiping it with anything...I guess the $1.50 at the coin op is more an insurance plan.
Also, you could probably do all this in the coin op stall if it isnt busy. Spray off real quick, pre-soak, fill bucket with wash nozzle, wash and then detail spray.
Basically follow Adam's winter detailing video, just skip the suds lol