Sustained over-voltage protection for a 1 MHz digital logic line
This is what I did: I inserted a 1SS404 diode in series with the signal. Signal from BA_SAFE to BA doesn't get too attenuated, and the reverse recovery time is short (~3ns). Everything appears to work fine, and if I unplug BA and connect 12V in its place, BA_SAFE remains unharmed.

Update: there is an important caveat here. What happens when BA_SAFE switches to LOW heavily depends on the diode type, and more specifically on its reverse recovery time (Trr). For example, with Schottky diodes that are usually specced at Trr &lt; 1 ns the only way the charge between D5 and BA can drain is through R32. If its resistance is too high, it can take quite some time, thus extending the falling edges and affecting the signal. Even at the relatively low 1 MHz it turned out to be an issue for me, one which I couldn't sidestep by decreasing R32 resistance (because then the voltage at BA would be too low).

On the other hand, if the diode's Trr is too long (and there are some popular ones that need tens of microseconds), the current drain from D5-BA segment will be so strong that its potential can go below ground: I measured around -400 mV for over 10 us with 1N4007.

As this was not something I was willing to expose BA to, I ended up picking up a diode with Trr = 50 ns (BAV19WS-7-F), which in my case is enough to preserve sharp falling edges without any measurable negative voltages.