It’s unlikely that the GDC would consider such a screw legal, see the discussion at Special partially threaded screw joints and HS gear bearings using modified 393 output gears - #12 by AperatureLabs. Threaded rods and eye bolts are not considered legal, so I doubt magnetic-head screws would be okay due to the additional functionality they have.
Yeah, I thought of this too. Nowhere in the rules (R21 at least) does it say that the screen protector is an electronic component: it’s not a sensor, controller, battery b=pack, reservoir, solenoid, anything pneumatic, motor, or microcontroller.