I don't know if you want to grind anything, but if you were to use a very small drill bit and a very gentle touch, you could create enough of a groove where the original slot was by drilling 2-3 spots directly next to each other. You will ruin the screw for sure, but if you are careful, you should at last be able to get it out with firm pressure and a sharp, square screwdriver.

The other option would be simply to NOT change the light seal ----it's probably okay, unless you are having obvious problems with the back. I've had pieces of the seals fall out of hasselblad backs (when I used Hasselblad) and never experienced a single fogged frame.