Through rigorous application of the experimental method, I have discovered that (at least on the C7) a valid backup of the firmware is only possible with hardware (HW) write-protect (WP) disabled. In this context, software (SW) WP seems to be irrelevant.
To clarify, if you use flashrom to read the EEPROM without bridging the WP jumper on the motherboard first, that backup copy of the firmware will be invalid. If you subsequently flash that backup (or a modded version of it) onto the EEPROM, it will brick the device.
Does this make any sense? No, but it appears to be a fact. Apparently, with HW WP enabled flashrom just silently (no error messages) fails to read the Intel Management Engine (and possibly other) code in the firmware image. I have found references to the fact that Google patched flashrom so that it would not crash under these conditions, but have no idea why. I would have preferred that it crash, rather than silently create an invalid image of the firmware. Of course, I must acknowledge that these tools were never intended for use by the consumer.
With this knowledge, I have successfully enabled the Dev Mode Boot Screen bypass with the stock firmware. So I can now confirm that "the hack" can be performed safely, as long as you keep this fact in mind. But, be aware that some of the instructions on the web do not take this into account and if followed to the letter, will brick your C7. I recommend only this source:
John's information was essential in my effort to de-brick my C7. He's using a Samsung 550 Chromebook, but the platforms are similar enough in this case.