Over the weekend I try how to turn on my computer via alarm Aux output. Well it worked as I expected. I have Mastero SDC.
My alarm aux output channel is (-) trigger (I think most are) and are programmable/selectable output duration (Continuous/pulse or whatever). Mine is 3 second pulse. So I wire the aux output to a relay and have a relay output (+) for the SDC ACC input.
Note: I did put 2 diodes one inline with the line going from relay to SDC and the other diode is inline with the car ACC line to SDC. I did it just to keep the car ACC and alarm aux/relay lines from messing up.

1st try. When I pressed the alarm key fob, the SDC turn on and PSU and computer. Cool. Well, the computer can only run for 10 min then shutdown.
1nd try. Before the computer run its 10 min, I put the key in the ignition. The computer keep running.

So basically the SDC will turn on when the ACC line get hot (regardless of how long the ACC line is hot). It will run till the preset timer is expired (cap discharge) then send power down signal then cut power.

I believe you can have central lock turn on the computer like this way when unlock.