It took some doing, and was a GIANT PITA, but I've got rear brakes now.
After I got the end seal squared away by pressing the metal ring into the master bore with the tapered end inward, it was time to re-install the rear master and bleed the system one more time... and pray that it would work.
Using a syringe with a rubber taper tip that came with my Mityvac kit I forced brake fluid both up from the caliper and down from the master reservoir. I left the steel pipe fitting at the master loose for the initial bleed to allow air to escape from both ends of the system.
The rear wheel was off to allow easier access to the caliper and to prevent accidentally getting brake fluid on the wheel or tire. Caliper was swung up to get the bleeder at the highest position.
After I tightened the steel pipe fitting at the master I pumped the syringe up and down on the master reservoir port to force any remaining air out of the master. This produced quite a few bubbles, making me believe I was on the right track.
Next I did a few cycles of the traditional pump the pedal and crack the bleeder screw.
At this point I had a good solid feeling brake pedal, so I put the rear wheel back on and buttoned up the system.
After pumping the pads back out to the brake rotor the pedal seemed to have a very long stroke, perhaps 3 inches, before the brakes would engage.
I decided to leave well enough alone for the day. Before retiring I blocked the brake pedal in the down position to allow any remaining air in the master to escape overnight.
The next morning I was pleased to find that not only did I have a solid rear brake pedal but the pedal stroke was now about 1 inch. HALLELUIAH!
Ya baby, I'm a BMW Rider now