I'm not sure if anyone on here has done this yet, but I know there was a couple of people planning this on Digital Car (not sure if I'm allowed to say that on here).
I'm going to be putting on in my Mini Clubman in the next few months. But then my mini won't exactly be factory spec!
You could put something in either of the rear compartment bins (but airflow might be a problem), or, with a van or pickup, I believe there are 'cubby holes' behind the front seats (underneath the rear seats in the van), so you could stash something there. As for where to put a screen and and keyboard/buttons, your best bet would probably be to get one of those flat dashboards, then you can either put a screen in the glovebox, or cut a hole in the middle of the dash. The buttons could go in a centre console.