In case anyone else runs into this issue without serial access, this indeed does work. I did it on a Voyage.
Better instructions:
Need a computer running linux. VMWare/Virtualbox didn't pass through the kindle interface correctly.
I used a 16.04 live USB image, downloaded and compiled (make linux)
https://github.com/TobiasWooldridge/Fastboot-Kindle, then used ./fastboot setvar bootmode main and ./fastboot reboot.
Nice find denydias.