No, I haven't, but did consider it once. The depth of the tray surface to the gas tank isn't deep enough for a sub box. It would work for a standard speaker mount though, like a 6" X 9", but you'd want to replace the thin masonite with something beefier for any kind of bass performance.
All that in mind, I can say from experience (my W116 380SE) that the tray itself presents no acoustic barrier to a trunk mounted sub, so the 2 trays on the shelf act like open ports. I get excellent bass from my trunk mounted Infinity Basslink.

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1986 560SL
2002 Toyota Camry
1993 Lexus