Great writeup! I plan on doing this with my Pioneer X9310BT (Canadian STi Sport-Tech) and an Apple TV for airplay. It has the same power requirements, and I found this simple DC controller which should do the trick, and possible even fit where the stock PS would be in the ATV.
http://www.amazon.com/dp/B008BHAOQO/ref=cm_sw_su_dp
I plan on hiding this inside the dash and wiring it to the deck via HDMI to composite cables. After that it's figuring out the software as I would like to do this without an internet connection... Worst case scenario you can set up a personal hotspot and it will work.