Can be done, requires jailbreak, video not so good
your ideas pushed my research in the right direction. Cables are available, but it outputs analog video only, and iOS only supports multimedia output for a few apps, not including Maps or other GPS/Nav apps. However, if you jailbreak your device, there are a few apps that will mirror everything on the screen through the output cable. A few have done this, but complained about the quality of the video output since its analog and very poor quality.
Supposedly it's better with iphone5 at it has hardware to support digital video out. Couldn't find any examples of actual implementation.
Android devices seem more flexible, and there were even a couple examples of screen mirroring over Bluetooth. But there is limited bandwidth with Bluetooth, so the mirroring wasn't smooth.
I bet we see more vendors growing the linkage between smartphones and car AV. Seems like lots of folks hate their internal Navigation and want to use their phones anyway.