Speaking as someone who's driven through a door...
Your policy sounds a little bit on the overkill side. IMHO, the backup spotter can (should be able to) do both jobs. We've got cheater stripes on the floor, so the backup man is more of a safeguard against the random idiot factor.
What about installing the "kiddie sensors" (photocells) at the appropriate height for your rigs, instead of 6" off the ground?
In actuality, our backup man protocol was initiated after we had a guy back into a POS nissan PU while 3-pointing the rig at a fire call. My only backup incidint involved a chain link fence getting lost in the mirror - no damage to the rig, but the post got keeled pretty good. Our department had been pretty good in terms of accidents before that, but a few of the safety folks jumped on this as an issue
.
You know, the same folks who complain about exhaust noise from the engines, but fire up the saws for morning checks inside the bays w/o muffs...
And the "through the door" incident? In the time it took for my officer to climb on board the rig and put us in route after I opened the door (about 30 seconds), the door catch failed and the door dropped 2 panels - just the right height to get snagged by the bucket of the tower.
Luckily, one of the officers on another shift had seen the door drop before, and had flagged the door on the "repairs needed" note sheet, so Tango wasn't out the deductible for a new door.