Well, the reason for the ceiling mount. It is accessible to both passenger and driver. I really don't like anything obscuring the view over the nose. I do a lot of off road, it seems that anything you put on the dash either is in the way blocking vision, or moves around soo much as to be unusable. Personally, I am used to getting info from the overhead, common spot to mount equipment in an airplane. I don't need to stare at it very often, I am not shooting approaches with it or anything like that. It folds up and out of sight, a good deterrent to theft. If you put anything on the dash it is like in a display case at a store, especially in the Syncro. I do have an Etrex Vista or I use the Garmin 295 I mount up there in the factory mount. I have found that they do wag a lot off road.
Glovebox mounting. Almost all the connections I need are there. There is loads of room in front of the glovebox for power supplies, hubs connections to GPS, stereo and access to the A pillar. I can also still actuate it with the IR remote, the signal makes it through the crack. You can actually pop it open an put a DVD or CD into it while in motion, by sight, not feel. I also do not think it is actually high enough for the mini under there. I do have Recaros in mine and the adjustment bar would also make access harder. I do remove the Mini from the van when I don't use it, and it needs to be easy to access.
I am making up a mount for a 7" Lilliput to put in my Range Rover. The Mini will go in the glovebox with it as well. The 7" fits perfectly where the factory POS navigation system went. I need nav that has contours, not just roads. I am a geologist, that does remote sensing with aircraft and I do go in the field to do ground truthing. The instrumentation does lie.
As for power. I tried my own design at first, but it was too noisy, it put noise lines in the video. So I took the path of least resistance, the P1900 power supply. I do not have it wired to do the turn off and on of the mini at this point. I do it with an external switch. I did make a 5 volt power supply for the usb, but it was easier to just add the third output to the P1900. I do run the lilliput off a relay driven by the secondary power output. I put a 12.5 Volt power supply at the head unit That way there is less load on the P1900. It is power output limited, and I am running the 8.4" monitor, which uses more power than the 7".
If they still made the MacBook Pro in the 12" size it would work well. The only problem I had using the laptop was you had to turn it on with the lid open, then close it and put it away. The 15" was a little awkward in size to put away. I have a 17" Macbook Pro and it is way too big to use in the car, especially off road. I have had a laptop do a screen-ectomy once in the past, a financial disaster do not want to repeat.