That would be nice, it would save pulling out 10 screws, disconnecting the battery, getting the fiddly connector onto the diagnostic port, triple checking the wiring, etc.
But now the secret is out, and its easy enough to do (and the equipment to do it is common place), it should take all of 20 minutes to fix Apple's stubborn-ness of not giving these machines the upgrade in lieu of making us buy even more upgrade unfriendly machines. Heck I could store a pi zero W, wifi pocket hotspot, usb stick and the flash cables in with my usb -> sata cables that allow me to look at other hard drives, and my HDMI cable to plug the machine into a bigger screen. The power adapter for the mac is more of a nuisance to carry than those items. A power bank in my backpack can power the pi when its needed, it'd provide plenty of juice for flashing the bootrom. Mod the bootrom, SCP the modified file to the PI on the mac, then a quiet spot located to pop out the screws, dislodge the battery connector, plug in the pi, and use the ipad to SSH in and run the flash commands (that could theoretically be scripted).
A software solution would be good though - save a lot of messing around.