Reuse existing intercom wires for low voltage power source?
Not code compliant. Use DB9 connectors to attach the USB to the existing wiring. Intel's white paper on USB Drop & Droop http://www.otdl.com/VDROOP.PDF Do a search, you can find some info on building your own USB charging circuit, vs. trying to find a power supply that is non-compliant.
Still think that it is easier to just pull up the 120vac wiring and/or just lay the unit on a table to charge. As for question before, what are the docking stations & tablets that you plan on using for this?
__________________ Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?