I have mine out there but I didn't go "through" the vent. The gap between the vent and the windsheild is very easy to tuck wires into it and they stay there pretty well. You run them over to the edge, down behind the trim, and loop them under the dash.
My main GPS antenna is mounted on the roof under a "radome" with a through-hole mount (in other words, I drilled the roof). However, I have two "spare" GPS antennas on the dash at the base of the windsheild (one for 3 volt, and one for 5 volt systems -- the 5 volt one has a lot more gain) so I have alternates for any GPS. Two different connector styles also.
So, you may be able to do something nice without even going through the vent.