The ethernet connection on the Chord is for receiving data. The power link sockets on the Beolab 18s are for a proprietary B&O audio input - they just coincidentally use the same type of socket.
You could connect an audio only device to the toslink input on one of your beolab 18s (and then another optical cable from the toslink output of that speaker to the toslink input of the other) and operate that device with the Beolab 18s for stereo music completely independently from your AV processor*, however any device connected to the Beolab 18s must have its own volume control.
*if both your audio device connected by toslink and the av processor connected via analogue are playing simultaneously then there will be an order of preference, but I don't recall what that is.