that thing is for network attached storage, and depending on the model you looked at it might have been for backup only--ie, you can't reliably read off it.
If you put that thing in your car, you would need a hub or a crossover cable. I think they act as servers, but I don't know if you could do it with a DVD drive. I know you couldnt do it with a keyboard/mouse. Ethernet is not meant to connect peripherals this way.
btw, if you did do it with a DVD drive, I remind you that you would have a 100mbps connection, which with collisions might top out at 60 or 70. This would be fine for hard drives, but maybe not so good for DVD. USB2.0 is 480mbps and is far better suited for this purpose.
ps--there is nothing preventing you from using ethernet cabling to extend your wires