I am trying to do the very same thing. This is actually a fairly complicated thing it seems the more I look into it. I myself do not have the ability to perform a remote Wake-On-Lan as I do not believe my motherboard (guts of a Sony Vaio Notebook) support the feature but the on-board NIC does, but I don't think the smart guys at sony made the connection leaving it as a broken feature or something along those lines.
Anyway, I then tried to do things like having a thin-clinet software load before windows, could have this in a thumb-drive, floppy disk, or a smartcard and consume very low cpu cycles and turn off hard disks and other things as it runs off the boards NIC. This could await for a command to reboot the computer but it consumes the same amount of power as if it was on normally so screw that idea too.
You cannot wake a computer from sleep or hibernate using Remote Desktop, so that will not work either.
Now I am back to looking at documentaion on my motherboard and WOL trying to see if I missed something the first time I tried it. But I am still basically ****ing in the wind. This is a very important feature for a car computer I believe.
Anyone else have alternative methods to remote boot?