I don't post here very often, so I hope this ends up in the right place. I have an 04 Civic EX coupe, exact same dash setup. I tried a clip on vent holder for my radios and the Civic vents just didn't seems to work. The placement wasn't quite right, the radio bumped into things, etc. Plus it scratched up the vent.
I foud out though, that the little flip down door just below the stereo is perfectly made to hold a handheld using the beltclip. I routinely clip my BC250D and my HS200 there. There is just enough pressure to hold the radio securely and it's the perfect length for running a power cord from the power outlet. Just pull the door open a bit and push the radio clip on and presto!! Just be sure there aren't any screws that will scratch up the plastic door, that happened with my IC T7H handheld. As far as a mobile, I don't think there's anyway it could be mounted without using a remote head.
Brent Toth
Brent Toth