I have a 2008 STi with a CarPC that uses the Openport to talk to the ECU all the time as I drive. So I wanted a way to leave the openport plugged in and have it switch with the ignition. Turns out it's easy.
The 2008 STi's switch pin 8 on the OBD II connector with the ignition switch. The Tactrix uses pin 16 for power, which is always on battery power as defined by the OBD II standard, so it never turns off.
This works great for me. Not sure if it will work on older Subarus or different cars. I do know that either OBDII or the USB will power the Openport, so even if I have the carpc on without the ignition on, I can still talk to the Openport's SD card (but not the ECU since it's not on obviously)
I wanted the Openport powered by the switched power, so here's what I did:
1) I opened up the Openport outer box
2) Cut the header leading from pin 16 to the PCB
3) Shorted pin 8 to the old pin 16