Unlike digital modes with control channels and parity bits, it would be pure analog. Not impossible, but the glitches causing risk of a failed transmission are likely not worth it. If you listen to AM with squelch off, it is a swirl of signal and noise. DSPs could do some filtering, but again adding a big risk factor.

Say plane A backs off and retransmits right when plane B transmits for the first time and plane C is on a path to collide with A. C might still suffer the same fate.

There really isn't a protocol. One can build an AM radio with a Zener diode and some common electronic parts.

Not trying to internet neg on the idea, to be clear. Just statistically, adding variables equally adds potential problems.