Don't know if you ever solved this, but I recently had the same problem. I used my Dremel to shave a milimeter or two off of my 1.4mm screwdriver blade, and it was just enough to allow it to fit into 2 of the 3 sides of the slot and take out the screws.
if you could, please tell me what type blade of screwdriver you modified to remove those screws. I have the same screws and need to remove them to replace a display from a non-working camera to a camera that works. I will gladly sacrifice a screwdriver to get those $%^#% screws off! you can e-mail me with your response directly, if you choose to. My e-mail is email@example.com. Thank you, and as we say here in Hawaii,
Mahalo Nui Loa !