Yes, that is exactly what the CarPC EZ power controller does.
The reason people use dc-dc power supplies for their systems is that the 12 volts in your car is not regulated, and if you use a standard atx motherboards or 3.5" hard drives, you need regulated 12volts. That is the biggest problem with PCs in the car (if you don't want to use an inverter). Well, the VIA systems don't use the 12 volt line, and neither does the laptop harddrive, thus you don't need a regulated 12v line. Likely, there is a dc-dc psu in the carputer case, something like a PW-60 that creates the 5v and 3.3 volts the pc needs to run.
Sounds like all you will need to do is install everything and you will be ready to go.