Trinitron tubes don't have shadow masks, Patrick. They have aperture grills. It's not a perforated metal plate, but a series of very fine metal ribbons.

I don't think that there is anything in there rigid enough to move in such a way.

I used to work at a monitor repair shop, and my brilliant boss decided that it would save money to buy video tape erasers instead of actual degaussing coils. The shadow mask tubes were simply permanently discolored. The Trinitron tubes had all the little ribbons magnetized, and they ended up sticking together. It was neat. (I'm still not sure why they would all get stuck together instead of being repelled from each other, but what do I know?)