Yup....I still have a pad of these in the center console of my '06; but, we are assuming these valet "jockeys" know how to read. I NEVER allow a valet to park my car. If it becomes an issue (due to their insurance, etc) I have them ride in the passenger seat while I drive to the parking area. It's a shame, but it is what it is.
What if they have to shuffle cars around while your gone? That's a risk in lots that stack up their area and fill every square inch. I like your idea and am going to use it....except in lots that shuffle cars.