Its not a BAD idea, just a lot of people have been waiting because thats an expensive way to do it. Yes, in theory that would work, but I don't know if you will be able to fit a different LCD into the top half of a head unit from (insert company here). Also, it's possible that the unit won't function properly without a screen that works, but I doubt it.
Also, you will NEVER be able to fit the controller board in the top half of a screen/ dvd combo. You will have to extend them to wherever you manage to fit the controller board, and make the cables flexible, which usually means hard to find cables. Then you will have to build an enclosure to hold the controller, and still fit that into your dash, very close to the screen unit, since the cables cannot be extended very long.
And it in no way removes the need for a computer. The computer gives you endless possibilities of stuff you can do, and the radio is just a conveniant method to hold the screen.
And it would go like this. Remove screen from headunit, replace with modified lilliput screen, run cables to controller board, making sure they don't interfere with the motorization. Then the video output of the unit would go to the aux input. Possibly, you could decode where the video signal was sent through the unit itself, because without the built in screen, you will lose ALL menu items from the unit. I don't remember any screens that displayed the OSD menu on the auxiliary screens.