Is your tone generator POWERED by the 120 volt line, or just safe to connect to it? Does your tone generator use batteries? If so, it must just be protected against damage from the 120 VAC line. If that is the case, you should be able to safely reduce the voltage seen by the toner with a capacitive/resistive voltage divider and use the capacitor to couple the low RF from the toner into the line.
The capacitive reactance of a 0.001 uF capacitor is about 2.6 megohms at 60 Hz, but is probably very adequate for coupling the RF from the toner into the line. (Bear in mind that I don't know the impedance of the toner or the frequency of the toner.) A 47K resistor shunting the output leads of the toner will assure that the line voltage is attenuated to a very safe voltage for your toner no matter what its terminal impedance is.
So I would suggest connecting a 0.001 uF, 500 volt or more capacitor in series with a 47K, 1/2 watt resistor from the 277 volt hot line to ground at the light fixture. Connect the toner ground, then clip the toner hot lead to the junction of the resistor and the capacitor (very carefully). This sequence minimizes the likelihood of coupling a transient voltage spike to the toner via the capacitor suddenly connected to the line at the peak of the voltage waveform. Check that the tone is present on the wire several feet from the toner. If it is, go find the breaker with your sniffer.
Be aware that I am not an EE, just a hobbyist. You take responsibility to make the connection to the hot line safely.
Careful!
awright