I see where the OP is coming from. If a buzzer goes off, and the only way to shut it off is to get out and throw a switch at the back of the bed, then at that point there is no excuse to "forget" to strap down the load. To be any more foolproof, the cutoff switch would have to be part of the strapping down mechanism, but I suppose putting that much engineering into a one-off would probably be excessive.
I like the idea of an optical load detector, over something having to withstand sliding pallets. Something strong enough to bounce the beam back with a mirror, to save having to run wires on both sides. A laser, probably.