Beep!
Non-777 driver here but didn't
Lost For Words mention a:
clever nosewheel sensor to let you know if the stab trim is in the right place
This supports the 'slippery pallet' theory as troubleshooting a stabilser out-of-trim condition would have revealed the load error prior to takeoff.
Also it seems like an aft displacement, the direction an unsecure load would shift at the start of the takeoff roll.