In a live session, Boot Repair refused to see the device, and as I said I was able to bang it out with two commands — No need for a PPA or installation of extraneous tools. I tried multiple formations of the media to see if that was the issue — it was not.
Boot Repair only seems to function on media which already have Ubuntu and Windows on them. It's a dual-boot fix'r up'r for people who didn't bother putting GRUB on a separate USB stick like I had, because I want to let Windows be Windows and do the Windows things without constantly worrying if an update will break my instance of GRUB.
The way I get around that is to effectively use a USB media as an ignition key. Once it's going, the stick can be removed and the system operates like I need it to. Update to Windows? Upgrade to 10X in the future? No worries since GRUB is completely isolated and air-gapped.