I would like to share the additions I have made to my Picade with a 12" screen. Thanks must go to @Dyltone for the button and voltage conversion ideas, and to @jopamo for the 12" screen and driver board suggestions, all from this thread: http://forums.pimoroni.com/t/12-1-inch-display-question/3262
My modifications involve two power buttons, the new Picade HAT, and a single 12V power source. All links to where I bought the items are below as well.
The key features this setup offers is the ability to have a single power source with a master on/off button, and an additional button that utilises Picade HAT's Pi on/off feature. Both buttons light up when on for a nice visual feedback. Also, my setup allows you to have physical volume up/down buttons (missing from the HAT), and the wiring of both speakers together. A full circuit diagram is below too.
Here are some pictures (yes, not the neatest of mods):
Here is a video showing them in action:
Shopping List (all items either shipped to or bought in the UK)
Hope this helps you have fun with your Picade too!