I'm already aware that the max allowable GPIO voltage is VCC + 0.5 V. Knowing these facts is one thing; it's another to put them all together and extrapolate a conclusion like what scenario a Schottky to VCC makes sense, especially when it's different from the scenario I was talking about originally.
If I put a Schottky diode leading to VCC and the MOSFET fails shorted between gate and drain with no zener to stop it going higher, then the VCC is 12 V and the MCU dies anyway? So I guess I'll keep looking at optocouplers.