transmitting data over powerline

This is how the famous X10 consumer product works. The frequency used is a few hundred kHz or lower. There are multipath problems on home wiring because of the star nature of the circuit. There are interference problems from conducted EMI from power supplies.

To avoid these problems there is a direct spread spectrum standard that is used by some industrial groups that make these products.

Take a look at w*w.pulselink.net that is part of their development path.

Powerline transmission uses generally capacitive (parallel branch) coupling. Inductive coupling (series branch) would depend on a low impedance load connected at the line end to generate a RF current.

In a multi phase power distribution, phase couplers (capacitors) are needed to achieve a transmission to any power socket.

Also power line modems for data networks are available, but the performance is generally poor, to my opinion. They effectively have been defeated by 2.4 GHz WLAN.

What would be the configuration of these capacitors in relation to the power line? How exactly does the signal get onto the power line?

Is it anything like:

-----Power-Line---------
            |
            C
            |
            ---------osc

Be sure to use a capacitor that is rated for use on the power line. Otherwise you can have a failure that damages your circuit or worse, your body.