Yes, you can do what you suggested, as long as the soap is thin enough to draw up the tube properly. The problem is most car soaps are very thick, and you need to dilute them to get them thin enough to draw properly.
I try to make it pretty simple. For a soap that has a recommended use ratio of 1 oz/gallon, I dilute it 3:1 in the cup (if you have the 32 oz cup, 24 oz water, 8 oz shampoo). Then I set the gun to 4 oz/gallon (middle hole) done. If you want a little heavier mixture, slide the bar to the 6oz/gallon hole, if you want a little more watery, slide the other way to 2 oz/ gallon. If you've got a thin soap like the Griot's, just put it in the cup undiluted, although since the usual dilution for that is 1/2 oz/gallon, you might want to cut it in half and use the 1 oz hole.
I usually use an old soap bottle to mix up a half gallon or gallon of mixture, so I don't have to stop and play scientist in the middle of a wash.
