I just wanted to put this here in case anyone else ever needs to do the same thing:
I managed to mess up my custom firmware in such a way that it would constantly spam out keypress events as soon as I plugged it in; this made it hard to use my computer with my Model 01 plugged in, yet I needed to have it plugged in to upload a new firmware. What I finally ended up with was to put this:
A simpler solution that has worked pretty well for me in this situation is to unplug the Model01, run make flash, then, when prompted to press enter, I hold down the prog key, plug in the Model01, and quickly tap enter on a different keyboard. This usually starts flashing the keyboard before the bootloader times out, so I don’t get any input from the Model01.