pichlo,
great idea, I did something similar, think I ran a script giving me the charge mA. but it is a problem when you want to charge different devices and I had a problem of knocking the phone off of optimal charge both day and night.
The best charging experience I have had was a drop in charger for my old Sharp SL-5500 Zaurus, what a great handheld Linux PDA/net tablet/computer. We could do that for the N900 with a 3D printed cradle, though it would require portrait mode for the N900 in normal gravity, it would also require finely aligning the microUSB tip as a bad alignment would threaten the USB port if dropped in too casually. I guess a cradle or tray and the Qi charge mod from this thread would work too, but my experience is that the mod works but is too fragile for everyday use. OTOH a drop in for a Qi enabled phone seems pretty reasonable, especially if it includes a nice BT audio amp also running on mains or automobile power.

I had actually considered such a two sided video-out/microUSB pwr clamp arrangement with a N900 for video out in the car to have a better screen for GPS-nav but never even tried to build it. I might still get around to doing something similar but installed for openstreetmaps with a Pi zero and a touchscreen.