I use a relay to turn on my inverter that runs off an ignition wire. When i turn the key my screen comes on up front (which is powered from the computer psu) and i know the inverters on. If i listen closely, i can also here it beep.
If you do this, pick a wire that is hot in run and start. Your starter will probably kill the inverter anyway, but atleast this will give it a fighting chance.
mp3dub, if i were you i would just run wires up front from the mb for power on and hdd.