Adding a light in parallel to existing loudspeaker circuit

I second gophert's like of the DC sent on the AC coupled audio line.
Reasonably simple to do and requires no added wires.
For response down to 50Hz, 10uF in series with each 100V speaker transformer should be adequate.
The capacitor at the amp output should be abou 10uF times the number of speakers connected to the output.

If you deciede to run wires back to the control unit you could look into using a circuit to do the whole lot over just 1 pair of wires.