I got the laptop I have for free with a cracked monitor. So I figured I would make use of it.
Why don't you sell your laptop, and get a small voltage one like a EEEPC or something. That's what I've got. It pulls 9.5V off it's car charger. My lighter adapter wire is only on with the ignition. I am using ShutControl to hibernate the PC when it loses A/C connection after running SyncToy to sync my media drive if my home PC is near by. It stays charged at 100% unless I sync up like 100Gb or something crazy.
I just inherited a travelmate 7520 with a smashed LCD. The screen is long gone but the body works great so its going in my car. I am facing similar power issues and have come to the conclusion that I will set up a switched connection (acc), wire up the DC to DC brick that I am buying to it, and run my laptop body as such. When the laptop senses power loss at the "brick", it will timeout in say 10 minutes and go to hibernate. Only thing to do is rig a power switch to the laptop body itself for remote start, or use a USB power solution and problem solved, yes?
If you only knew the power of the dark side.....
Assuming the above to be true, you would need to tap the 2 contacts going to the power button on the laptop and [with the correct polarity] connect those in parallel with the PS's motherboard on/off line. Of course putting that in parallel with I assume a new momentary switch mounted elsewhere. This will let the DC-DC PS send the remote on/off signal automatically, and give you manual control when the PS is on as well.
i think it would be easier to just set the laptop to shut down after 5 min of being on the battery power, and have it auto-wakeup upon ac power. if you can do that (depends of bios), then all you need to do is put a switch inline with the power going into the laptop... and also that way, when you hit the power button you cut all juice going to the laptop so your car battery is saved.