This is an awesome idea and a great thread, and you're making good progress.
Turns out in car pc's are are a moderately popular thing.
I'm not sure if I'm adding to the discussion or not, but have you seen this:
rcjoycon
It's a steering wheel audio control to USB converter. Our car isn't listed in the compatibility list, however the the 350Z is. I'm not sure if we have resistive controls or not, but looking at other after market controllers the 350Z and the 370Z use the same controller.
The adapter and configuration software is $46 on ebay, $6 for shipping by slow boat from South Korea and ~$23 by EMS.