Im sure someone has asked this type of question but I have not been able to locate it.. Im PRETTY sure the answer is "NO" but, if you dont ask...
  OK, here is the situation,  My Betafpv TwigXL finally gave up the ghost.  Reason I say this is a few years back my buddy ordered 2 of these by accident, he asked if I wanted to buy the other before he sent it back.  After seeing the resolution of the Caddx ratel I was SOLD.  Betafpv component quality wasnt a major issue at the time as components were cheap compared to now.  Anyway, his FC / AIO was taken out by some GRASS..no joke.  So, I located some new AIOs, he installed his and I just WAITED for mine to fail...well a week or so it did, but it was not by grass, it was a light pole I hit at top speed...
   I looked at the board and a component was shifted / moved... I re-soldered it hoping it was power the esc, no luck.  The FC part of this AIO is fine, no issues.  its the ESC part.. so here is what I am asking
  Can a ESC be installed on a AIO with a "good" FC and a failed ESC.?? 
  Im guessing NO

Hi Rob,

Well...maybe it can be. What fails is generally a Mosfet or FET and these actually ARE
replaceable. Let me see what I can find.

....well, apparently the "repair" i did worked. After resoldering the cap / diode / resistor (whatever it was) I added power and did NOT get any tones. Then I realized I had no motors soldered to the AIO fc... cant get tones from the ESCs if there are mo motors...
So I added a motor got all 5 tones... reinstalled, and SEEMS OK. When I was testing it (indoors) it would drop / failsafe. Now, I do have a XM+ in this and know when flown close to the transmitter this can happen... i dont think it is the esc failing as it doesnt 're-set' (starting tones),,,

Sounds like you don’t need this answer anymore, but…
I think the short answer would be yes, but not easily.

You’d hafta find a good place to get your signals from- probably the mcu. You’d probably want to stop the onboard esc’s from getting power or at least signal via scratching through some traces and removing some components.

It’d be a lot more work than just identifying the component/s that failed and replacing them.

you need a "timed" source, led is a timed pad for sure, you can remap led pad as motor