That's how I think it should look. Using the relay on the ignition to short the com pin 8 and com pin 4 together for shutdown using the nice little program discussed in
this thread. The capacitor than loses juice over 30 seconds - a minute or so (enough time for shutdown) before turning off the main power relay for the inverter (hard power down for soft failure and to switch inverter off to save battery).
Laidback- I still don't understand how 5v from the psu would shut the computer off when you switch off the ignition. You would have to go from an acc line or ignition line to tell the computer to shutdown (assuming your using the fake UPS idea, which i substituted with the com 4, com 8 short idea in my latest plan).
Also, would it be better to just attach the first relay to the actual switch of the inverter, instead of inline with the power cord? For example opening the inverter to get to the two leads that are connected when the inverter is on to use for switching.