Nope, I haven't seen a solution like this either! But I'm going to build it this summer!
I already bought a 12" PB for this project and I'm currently ordering the rest I'd like to use: 2nd car batt, Isolator, InDash-Screen, power supply, wireless keyboard, Amplifier, Powermater etc...
I'm favouring the PowerBooks because of their battery, ruggedness for mobile use and the possibility to take them out and use in a hotel room e.g.
If you have questions regarding PowerBooks and iBooks, feel free to ask. I'm an Apple Service Technician. (But new to the 12V-environment...)
Let's put our PowerBooks to truly mobile use!