Take a look at www.hth.com.
They have a power-line modem family of products (PLM-24), based on an ST (SGS-Thomsom) chip.

I think that power-line is not a very dependable communications channel. You should not use it for a security system except after you have tested the system carefully. Use a fail-safe protocol.

Take into account that the system should (probably) operate under two different conditions: With the power-line o.k. and with the power-line de-energized.

Regards,

Alfredo del Rio.