So, if I understand correctly, this is what you want to happen:

1.) When you start the car, the Mac mini either starts up or wakes from sleep/hibernation.
2.) When you turn the ignition all the way off, you want the Mac mini to go into hibernate mode.

This sounds doable. While I can't help you with the fancy combination of start/wake and off/hibernate, I can help with the hibernation itself. Check out SmartSleep. It's a preference pane GUI for the hibernate command-line settings. You can use it to set the computer to always hibernate (instead of sleeping). This means that when you tell the Mac mini to sleep, it will hibernate instead. No "tricking" necessary. Just some changing of settings.

Before you use SmartSleep, you'll need to run this set of commands in Terminal:
sudo nvram nvramrc='" /" select-dev
" msh" encode-string " has-safe-sleep" property
unselect
'
sudo nvram "use-nvramrc?"=true

I suggest you restart, then install the PrefPane, then see if it works. If not... Well, it should have worked.
_________________________
Cheers! :-)
- JediJoker

Current:
- "ProBook:" 15.4" MacBook Pro Core 2 Duo 2.33 GHz
- Mac Classic II
- Old PowerBook

Moved On:
- "The Book of Power:" 12.1" PowerBook G4 Aluminum 1.33 GHz
- Purple iMac