I'm a little surprised that you wouldn't be able to just do it on the carputer. The greatest utility would be to be able to bring up the address in the web browser and then hit a "nav" button that would have IG navigate to that location. Or maybe there would be a button that says "Save" and it would save the information to the ini file on the carputer. Your application could then give you the option to navigate to it, map it, or whatever.
Maybe I'm the exception, but I never do any trip planning on my desktop and can't see it being useful if I have to save the information to a file on my desktop and then port it to my carputer in some manner. As you know, I'm not a programmer so maybe the ideal is simply not doable.