Go very slowly down hill.

The brakes themselves are not likely to be the failure mode, you'll overheat the rims and have a blowout, and/or overheat the pads to the point of fade (and lots of stink).

A spoon brake is not going to help with the overheating of the rim issue, and will most like result in overheating the tire as well = blowout sooner.

If your rear hub has provision for a drum, a drum (hard to find) or disc could be mounted. The threading is 1.375 inch by 24 threads per inch, same as a freewheel, and the same hand as a freewheel. You'd have to fabricate a bracket for the disc brake caliper. DT Swiss makes an adapter to mount a disc, which several internet dealers in the UK stock.

Discs are not meant to be used as drag brakes; to keep the working properly, use them hard, get the speed down quickly, then let off to allow it too cool.

Your cantis should be used the same way.