Ok, I just threw a show_message("Tone Init", 2*HZ) in after the tone_init() in hijack_handle_display and the result is interesting:

On AC power, I always see my message pop up.
On DC power, I never see it pop up.

This suggests that tone_init is not getting called in DC mode. (It also suggests that any initial= line in config.ini doesn't get appplied.)

The only difference that I can see is that I get an "Info:Track" display momentarily after the visuals are loaded when in AC mode. This happens immediately before my message pops up, but I haven't seen it in DC mode.

Maybe this is related to DC visuals restore after all? I'm going to play with my DC restore settings in the car.