You could probably install an electric brake controller in about an hour. IF
(a super big if) I were considering doing it, I would use a motion based
sensor (I like the Prodigy). I would get it ahead of time and rig the power
to plug into the lighter (with a fuse in-line). I would also buy a female
seven pin connector for the tow vehicle which has a separate plug for the
standard four pin connector (you just plug it into the 4 pin connector and
the wires for the lights on the seven pin connector work). You would need
to run a wire from the cab of the truck to the back for the brakes (you
could probably duct tape it along the outside side of truck). The only
permanent modification to the Uhaul truck would be drilling two small holes
under the dash for the brake controller mount. You might even be able to
use heavy duty velcro, but it needs to be secure. Proceed with this plan at
your own risk (and out of sight of U'haul).
Jarrod