I like the idea
It is just a matter of placing the unit in the car in an out-of-the-way place. I'd vote for a location just under the stereo.
I'd not prefer the sun-visor location, simply because of the nightmare that routing the cables would cause.
You are going to use the UMPC for ICE also, right? So you'll need to manage a minimum of 2 cables (power-in and audio-out). You can always use an EP to RCA connection to route the audio-out of the UMPC into the aux-in of the car's HU.