Just check out the tatra mod for the interface, if you ask the guy nicely enough he'd probably release the code.
Another thought that hit is why not suspend the mini in the standard stereo slots from like rubber strings. Then you just stick a small TFT or LCD in front of it to cover up the hole. Perhaps even add a aluminum frame or something.
With bluetooth enabled mini coupled with wireless mouse and keyboard you don't have any cables to worry about.
And most of the time it's just an empty hole down to the firewall behind the dash and it's cool enough. Worst case you can always chuck a 12V papst 120mm fan behind there and just hook it up to the 30 rail (standard 12 volt on ignition power). If a normal psu can handle it so should the cars electrics.