I downloaded openpli-6.2-release-vuzero4k-20180611_usb.zip from openpli.org, then unzipped it and copied the resulting vuplus directory to the root directory of a USB key. (It was a 2Gb, FAT16 key and I read later that it should have been 4Gb+ and FAT32, but it worked). Turned the Vu+ Zero 4K off, plugged the USB key in the back, turned it on and went off to have some coffee. About 10 minutes later I figured it was either bricked or done, turned it off, pulled the USB key, turned it on, and was greeted with a big OpenPli splash screen.

A big thanks to the programmers of Openpli for making everything so user friendly -- I didn't read any instructions for setting up and customizing the receiver; just fiddled within the existing menus.