How to decelerate effectively with a rear brake? My front brake is not operational and it will be a couple of days before the replacement Shimano Deore arrives. There are probably other use cases for braking with a rear brake. For example, pad glazing or boiling fluid due to  excessive use of front brake downhill. Or sudden brake cable failure (has happened to me - the cable got cut at the caliper).

I have determined that my stopping distance is about 3 times that of both brakes operational.

Because the center of gravity of the rider is not located on the line between the two contact "points" of the bicycle with the ground, when decelerating an overturning moment is generated. The result is more weight on the front wheel and less on the rear wheel. More weight equals more traction or more stopping power.

Are there any techniques for effectively stopping with only a rear brake? Some way to put more weight on the rear wheel?

Brakes: basic Shimano hydraulic discs
Tires: Schwalbe Land Cruiser
Use: commuting and local autumn trails
Yes, there's a technique, however I strongly recommend you to fix the front brake. Rear only is dangerous.
The rear wheel tends to block much sooner than the front one because of reasons you mentioned in the question so the obvious way is to avoid blocking as much as possible. First, you can emulate ABS on the car by quickly pressing and releasing brakes in series, this way the wheel will likely block, but with less tendency to slide sideways.
Another is moving your mass center towards back while braking - not cool, but it helps, actually.
And again - fix the brakes, repairing them is not a hard job.