I'm trying to figure out a way to be able to use the Raspberry Pi to act as a syncing device between my home network and my car.

I'm imagining it would work something like this:

Car Receiver that has USB ports and mp3 support, probably the Pioneer DEH-X9500BHS

USB mass storage device, plugged into the receiver, provides music library, uses mp3 browsing featured built into receiver

Raspberry Pi w/ wifi, automatically syncs music from home server to usb storage

This assumes I'll be able to figure out how to provide stable power from the car to both the pi and the USB storage device. I'll have custom software on the raspberry pi and my home network to automatically sync music files when the car is in range of my home wifi.

However, the problem I'm grappling with is how I can connect the pi, the receiver, and the USB device. My first thought was to expose the pi as a USB host device, but based on this question/answer, that doesn't seem feasible right now. The only other thing I could think of would be using some sort of USB switch, but I don't want to have to push any buttons to allow it to sync - or have any of the extra hardware exposed, for that matter. Are there any other options available for this problem?