Thanks! I was wondering a little about this already, but I found the magnet is so thin that it doesn't normally dip all the way into the well in the bezel, such that it would touch the screen when the cover was closed.
Of course, you're not talking about normal conditions. You're talking about someone grabbing both case and Kobo hard, like a hamburger they're about to take a bite of. In that case, the felt sounds like pretty good insurance, and I may try it soon.
In more extreme conditions, such as someone sitting on your Kobo, it's probably doomed, magnet, felt, or nothing.