Sounds like a cool idea, but honestly too much work and probably $$ for me. I know you said you wanted to avoid the cable going into your phone so sorry for this post, but I figured I might as well be helpful to other people looking for a phone mount.
I just bought one of these Battery Tender USB Plugs
($10), a 3 ft iphone charging cable
($8), a Rokform Motorcycle Mount
($130), and Rokform Case
If you do go this route, be sure to get enough clearance. Turn the handlebars all the way and make sure it doesn't hit the tank. I forgot to do that and now I have a small scratch on mine. Also, the quick Battery Tender USB connects directly to a factory installed Honda cable that is connected to the battery. It's easy to attach. You just pop it on. I ran my cable under the seat, gas tank and up the handle bars. I used zip ties so the cable doesn't go bye bye when riding. The angle the mount holds my phone at is perfect for me.
The only downside is that the cable is always powered on even when the bike is off. (This can be fixed with a relay, but I didn't want to put too much work into it.) It shouldn't draw too much power from the battery from my understanding and I don't have it plugged into my phone all the time. (Maybe someone could give some input on the power drain of this method? From what I've read it's minimal). If you live in a big city and park it on the streets, it's possible someone might MIGHT plug their phone in for a charge, but I doubt it. Either way, I used some sugru to make a little housing unit for it when it's unplugged so it protects it from the rain although I'm not too worried about it.
I know it's not exactly what you're looking for and the rokform mount can get pretty price, but I like the aesthetics. It's less bulky than the RamMount and probably harder to steal. I have the RamMount and the Rokform is more solid, in my opinion, in terms of your phone not flying out of it.
Good luck with your project!