As mentioned above it's for tensioning the chassis to prevent rattle. I don't think it's for grounding. Think about it..... the chassis itself is touching the cab already. Why would a screw on some foil be a ground? The foil has no wire connecting it to anything else.
I think the foil is there to prevent the screw from digging into the wood too far. I wouldn't tighten too hard either, it's only meant to be enough to prevent rattle. If over-tightened, the screw could burst through the foil and you'd crush the wood. Over time (vibration), the screw would chew the wood and the chassis would rattle. You'll tighten some more and crush some more wood..... vibration>rattle>tighten>crush wood. Do this over and over...... Get the picture? You'd end up with a hole in your cab.
Also, before removing the chassis, loosen the screw otherwise you'd score the foil and wood while pulling the chassis out. You may even rip the foil.
My guess is Mesa use foil with a sticky back (ie. like sticky tape) already supplied to them because it's easier in manufacturing to place on the cab. A metal plate would be more robust but you'd need to apply a sticky backing to it, like double sided tape, which cost more $$. A plate may also be too thick (even 1mm) because sliding the chassis back in may damage the front edge of it.
If you want to get clever, use a router, cut a diameter larger than and to a depth of a large coin and glue a coin in. Instant increase in value. And you don't have to worry about punching a hole through it :lol:
