The value of persistence:

When I first ran the boot utility against a flash drive with stuff already on it, it wouldn't let me create a "Floppy Image", only the HP firmware flash device.

So after doing that, I went through these steps one more time, and it let me re-format the drive with the Floppy Image the second time without a hitch.

Not sure why it balked in the first place, but just FYI if someone is following these instructions and gets flummoxed like I did.