It appears as though you have several questions here, and I believe that a few questions need to be answered first.
I will attempt to answer a couple of the questions I believe you are asking, and I will ask a few questions, and answer guessing your reply.
To answer your first, obvious question, in my opninion, you should not use RO reject water to apply a high pressure wax. It will be VERY high TDS, and would be the same as applying with hard water.I would only use the RO reject water for a high pressure rinse assuming it was followed with a spot free rinse.
To answer the statement(question?) about guys mixing the RO reject with city water, I would think this would be fine for rinse water, but again, I would not run this RO reject water with any type of sealant, or wax.
To answer the third statement,( question?), I would not plumb RO reject water inline after the water softener to feed anything that would be applying a sealant, or that would be supplying water to the RO system, ( that would be after the water softener
)
I am all for saving money where I can, but there are (in my opinion), and there are ways to cut corners to save money. I would rather offer a better product( no RO reject water with wax), and charge a little more, and get the repeat business. Remember, you only have one chance to make a first impression.
To reply to your last statement,( question), The RO system should not be running if you don't have business. I would think of trying to use the RO reject water, if available, but do not "count" on it being there. I would make plans for " what happens if I run out of RO reject water" when you are making plans for your equipment.
I would simply use the RO reject water for HP rinse if it is there, otherwise it would pull from the main tank.
HTH