Hmm, just a thought, but maybe:

If I power the empeg via the AC connector, the player software doesn't support quite a few car features, like 4 channel EQ, phone mute, dimmer setting,...
If I power it using the sled, I don't get full speed serial (unless I changed config.ini) and I don't get the kernel upgradability through the firmware.
Now, with HiJack installed and powering via AC, if I tell HiJack to force DC mode, even on AC power, I should get all the good stuff, right?

• Kernel upgradability is given, because the firmware is active before HiJack is loaded, so it correctly thinks it is on AC power.
• 4channel EQ, tuner, dimmer etc. are enabled because the player software is loaded after HiJack and thus thinks it's on DC power.
• The serial is at full speed if I modified config.ini accrodingly.
  

As far as I can tell, that is the (almost) ideal solution. The only problem is that the serial port is also at full speed when in the car.

cu,
sven