Sounds like my "hoopty"
Tried every trick in the book "on this forum" no luck. I purchased some brake anti squeal from the MB dealer and I would apply to the inside of the pads, Yes on the braking surface and the squeal will go for about a week and then all of a sudden I get it back.
I heard that running over the pads with a rough sand paper sometimes does the trick, I have yet to try that, but I would imagine it would only be a matter of time before they glaze again.
I know exactly what you mean about the embarrassments of driving a squeaky S class, and believe me I am experiencing the virtuse of loud stereo in the car, in the winter it is not that bad since you don't hear it as much.
Did I mention I replaced the pads while they were new to get rid of the squeal and the new ones squeals after a few hundred miles
Good Luck, let us know what you come up with.