... Or at least walk beside ready to pull on the emergency brake. ...
Stopping a runaway is what biloxi was getting at when he suggested 'I also would drag a piece of wood placed in front of the front wheels and tied to the front spring hangers'.
This piece of wood should be of a large cross-section, say 6" minimum, and tied such that it is behind the front wheels going uphill, so that if the operator loses grip of the starting handle, and the vehicle starts downhill, the wood acts a chock, or sprag, against the tyres, to stop downhill movement. Enough slack has to be left in the rope to ensure the wood stays in place, and not so short that the wood just dragged forward in front of the wheels as the vehicle rolls downhill.
Of course, this sprag is in addition to the body of the operator being caught under the vehicle.
You do realise that if the operator pauses for breath, grip and pressure must be maintained on the handle to stop it spinning out of his hands as the vehicle starts to roll forward?
There is quite a personal risk involved.
The brownie points you are seeking to gain by hand cranking the vehicle backwards uphill will have to be earned; they don't come for free.
If you are going to have a walking emergency brake operator, the stop on the drivers door check should be disengaged and the door held fully open against the wing (fender), so the brake operator can easily and quickly access and operate the brake.
... A tad worried that you think your new cross-member might not cope ?? This might be the confidence builder you need ? ...
I share this concern. It terms of expected load in service, the pull will be quite low value.
Regards.