I've got one as well ...... and thought about it, but not investigated any further yet.
I haven't even got around to updating my sat nav maps since I did the initial update when I got the car in January (occassionally find myself driving in 'fields' instead of on the new bypass!)
Saying that, I tend to identify the caches I want to do in advance, so probably wouldn't want lots added on the car sat nav (will put them on Oregon instead). However, I do use the coords option on the car sat nav and manually add the odd cache/parking if in an area I am not familiar with roads etc and that works really well.
But interested to hear your outcome ....
GAGB member since 2005
GAGB Committee member 2010 to 2016 (Chair 2012 to 2015)
UK Mega Event Chairman 2009 (Weston-super-Mare)