If there are any sharp turns you could "miss" due to an overspeed or trailer push mishap, I would walk away personally. Rear axle slideout is one of the the biggest risks on a slow speed downhill turn with a bumper pull. If it was me, and my "dead reckoning" said it was possible, I would do two things:
Run as slow as humanly possible without skidding or sliding tires on the tow rig. If, and once, your rig begins to be pushed over your desired speed by the trailer, if you attempt to slow it by locking tires on your rig, especially rear, you are at high risk. It is better to limit the acceleration of the combo as best as possible down the hill and focus instead on maintaining directional control, rather than make hard braking efforts to slow something heavier than your truck - if any axle starts to move latterally (sideways) during this decent you are at very high risk for a bad, bad day.
Air down axles on the truck and trailer both. This increases rolling resistance which will give you more natural 'braking' force, it also helps prevent lateral movement of the tires on soft non-compacted surfaces. Aired down tires will also have less propensity to slide and/or continue sliding due to over braking on the tow rig.
Plan it out in your head first, all your turns and moves in advance, some people seem to switch off their brains when performing risky manuevers when they should be doing the opposite. Trailers want to go in the direction the tongue is pointing, no matter where the tow rig is pointing - especially down hill. If the pull looks too risky, you can supplement a straight tow down a mountain side with a rear mounted cable attachment to the trailer and a shackle mounted to a tree or another sturdy object on the pass. Another vehicle, travelling on a level roadway above the pass and at an oblique angle, can then pay out the cable in a controlled manner to the downhill apparatus. The upper vehicle should weigh as much or more than the downhill rig combo, it can be ballasted if necessary - this increases safety. I would not ballast (add weight) to a downhill pull rig in any way. You can use a smaller vehicle with a longer run of cable and a block and tackle between the solid object and the downhill rig, to increase your mechanical advantage. a large winch can perform the same basic task. Put on your thinking cap and work outside the box as necessary. Even the best laid plans can, and have, failed catastrophically against all predicted odds. Remember the Titanic.
