Personally I'd like a mini magsafe connector, maybe mini usb magsafe? Note sure if it would work when charging only from USB port though, maybe not enough current to drive the magnet?
The magnet isn't powered by electricity.
The real obstacle would be that a phone is far too small (i.e. not heavy enough) for magsafe to work.
You need it to be heavy enough that the cable will disconnect with a sharp tug on the cable rather than just causing the phone to fly off a table.