Originally Posted by dioxide
I second this suggestion, it should be built into the mixer.
I would add that this, that if this feature IS added it should be on the Control Room output ONLY. No physical mixer has a mono switch on the main outputs, only on the MONITOR outputs. Putting it on the main outputs (or using a Combinator) can result in the potential for accidentally exporting mixes in mono. Putting it on the monitor path (like physical consoles) ensures this can never happen, and that's exactly why this feature is implemented that way.
PLUS, monitoring in mono is best done on a single monitor, so you also need a way to cut one of the two stereo speakers when using mono mode (also common on physical consoles). :-)