I don't know if this is possible at all but if it was possible to route a different mix to the monitor output of the CS18 than to the headphone out, you could use the monitor out as a delay line, mp3 recording at FOH, etc...
The way it is now you wouldnt be able to do that because if you soloed a channel for the headphones this would happen to the monitor out as well and the recording would be unusable.