[Q] Fix or Workaround for No USB Access to Bootloader?

Is there a way of issuing fastboot commands to an Android device without a rooted device and without having a USB connection to a computer?

For example, I attempted to "convert" my (unrooted) HTC One from an AT&T to a Google Play Edition device with the procedure described here.

As part of the procedure, I attempted to flash the stock Google Play Edition ROM using fastboot commands but got an error message. Consequently, my phone is in the following state:

1. When it is fully booted, the phone operates fine.
2. When it is fully booted, I can connect to it via USB and issue ADB commands from my computer.
3. When it is in the bootloader, it fails to connect via USB, and issuing fastboot commands from a computer results in "waiting for device" messages.
4. When it is in the bootloader, the OS version info is highlighted in red, indicating some sort of problem.
5. When it is in the bootloader, any attempt to enter recovery mode does nothing but show an image of a phone with a big red X in it.

Any Android device could theoretically get in this state of "bootloader paralysis". Everything we want to do to regain control of the bootloader and fix the situation seems to require issuing fastboot commands from a computer connected to the phone while it's in bootloader mode. Yet entering recovery mode or getting a computer to connect to it via USB while in bootloader mode no longer seems possible.

FYI, the top of the bootloader screen shows:

Is there a fix or workaround?