John, do you know if it was ever the practice to do that on the D&RGW? My impression was that 'braking by Johnson bar' is quite possible, but hard on the motion and pretty much relegated to emergency situations only after air brakes (or even steam-jam brakes) became widespread.
I believe there was such a thing as the 'Le Chetalier' water brake, which was a special device that admitted a small amount of superheated water at the far end of the piston stroke and let the piston compress the resulting flash to steam for braking power. Its undesirable effects ranged from blistering the paint off the cylinder jackets (because Boyle's Law) to possibly launching a head or other cylinder failures, but that's the only method I've heard of for controlling an engine that way over a long descent.