Ok, well... just for the halibut, I installed a fresh copy of Sierra on an external drive and booted off that. The first attempt at running the Recovery Media Builder on a Kingston USB 3.0 flash drive seemed to stall out, so I force quit HDM Mac. There was no error message this time, but the progress bars had gone all the way to the right and RMB put files on the flash drive this time.

I decided to restart again and (still on the fresh install of Sierra) started the RMB again. Once again it seemed to stall out towards the end (it was slow overall, btw), but no error message. I let it go and it finally announced that the recovery media had been created. Elapsed time reported to create the media was 49 minutes 15.484 seconds.

Yo!