... would have liked to have videoed the belt while driving, to see if it tracked differently under differing load conditions. ...
That should be possible:
1. remove liner and boot lid - needed to get light for videoing. [Is the boot lid easily removed? Seems like it should be]
2. set up GoPro / Mobius or similar on a clamp mount on the frame. Settings should be highest frame rate (frame rate prioritised over resolution).
3. Optionally use a remote app (eg for GoPro) to start recording when about to accelerate...
PS: I note the chap who did the original M3W (see
Pete Larsen's Liberty Ace project ) used a driveshaft at the expense of passenger space/comfort. The latter could maybe be handled by a taller seat or forward set seat for the passenger...