Here's my thoughts. I would go with just a regular DC-AC invertor. You can get a cheap one that doesn't have a lot of wattage. Hook this up to the ignition.
-When the car is on, power will be supplied through invertor to the laptop thereby giving it AC power.
-When the car is off, there will be no power through the invertor so the laptop will run on its on battery power. If you're running Windows, you can set the power management settings to put the computer into standby or hibernate once the battery gets to 50% for exampe (depending on how quickly your internal battery goes).
The Opus and ITPS are more for desktop type systems. I don't think they will be needed for a laptop since you already have a battery with internal power management capabilities.