I think, i would be usefull to have a special version of the firmware for debugging purposes. At some point an ESC goes bad (from a crash or other reason) and it would be nice if you could debug the problem in-system.
That version when flashed to the ESC, should be able to help at least on these scenarios:

• test if the FETs corresponding to motor wire A,B or C are working. The firmware could apply different waveforms on a FET in order to check this with a oscilloscope, a lightbulb or other mechanical means (motor, speaker etc). For example a test could be "apply Vcc for 5 secs on motor-wire A" in order to check if the FET is working, with a lightbulb connected to the motorwire A and GND.
• test if the feedback (zero-crossing detection) from motorwires is working.

There could be other tests also and we could have the good ESCs (in a 4in1 ESC with one bad ESC) beeping to signal the test results.

Whats your opinion on this?