No, it's irrelevant which end is supplying the power and which end is transmitting the data. Quickly, for example as I'm under pressure at the moment...

Slave is a 16F628 (building block of the modern world!). HPWM an output at say 40kHz... hardware cross-connect that back into one input of one of the on-board comparators. Take Hardware Serial out of USART or Software Serial out of any other pin. Hardware cross-connect that into your other Comparator Input. Take Hardware comparator output pin via Capacitor onto your Supply Line (yes, the supply will need a blocking choke). This will then transmit your modulated data signal back to the Master end. At the Master end, take via a Capaitor (PSU will need a blocking choke as well) into a demodulator (which if you chose the values correctly could be as simple as a Diode and a Capacitor), take it's output into USART or Software Serial pin. In it's raw basic form it's not exactly going to pass EMC specs but who cares anyway! It's not noise immune either, but repeat the data five times with a check-digit. Anyone with a problem with this concept, go read basic AM radio theory.