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.