Serial commands sent to /proc/empeg_notify don't work if the player is started with the -s- option, which in all latest versions of Hijack it is. This is to allow userland access to the serial port, but makes it hard to tell the player you want to do something new.

See this thread.

Edit: Forgot to mention that Hijack only starts up the player with -s- while in the car, not on AC power.