Just figured I'd throw this out there, but with my proposed solution, you could use ANY tablet in existence... User preference... Every software ecosystem has some sort of VNC-like client, and a server that can be installed on another machine. Hell you might be able to do raw RDP--I know you can on iOS and Android. I am not sure if RDP can handle HD video playback, but there are other solutions that can. Personally I'd probably just play the HD video straight off the tablet, accessing my CarPCs' HDD over the network. For anything other than that, I'd use the CarPC.
I am hoping to have this come together in the next month. I'll have to start a thread for you guys.