X10 home automation sends signals through a home's high voltage wiring. It transmits only when the waveform is at zero-crossing.
There may be articles which discuss what components they install for safety.
A small value capacitor might be sufficient. Probably 2 capacitors, one between each connection to mains lines. Its value should block 50 Hz from getting to your circuit, but admit 30 kHz.
It would be nice to have a transformer do the job so you would have isolation. However I'm not sure it could have suitable parameters and still be safe to attach across 50 Hz high voltage.