Thanks for the info, that's helpful and interesting!

Thinking about it in more detail, and drawing out the circuit diagram above, I think I've misrepresented my requirements. I think I might just go with a low power schottky diode (like this, but sourced from a more reliable place)...

The current flowing through the diode during charging is actually going to be 6.66 (maybe 10 max) Amps (550mV drop voltage), which gives me an upper limit heat-wise of 3.6 - 5.5W to dissipate, which is more manageable if I give it a decent heatsink.

The current flowing through the diode during not-charging would be like, 500µA@42V = 0.02W.

Sorry for the confusion. For some reason I was stuck in a though-loop, thinking the diode would need to be rated for the max amperage I intend to discharge to/from the battery and ESC. But instead, I believe it just needs to be rated above the max battery voltage to prevent discharging in the unintended direction.