1- Throwing out an idea here: It might be better to have 2 independent headphone outputs, as opposed to XLR outputs. Some mixers like the Sound Devices stuff take headphone/mini-jack as the return feed, not XLR.
If you want to record sync sound, a reasonable setup would be:
Two audio persons. Each sound operator holds a boom and a single wireless lav (or handle two wireless lavs). They also handle their own levels and mix their feeds. They can monitor right off the camera (via the return on their mixer).
This works well with the Sound Devices mixers, since they can't output more than 2 channels. They can have 2+ inputs, so you can have a boom and at least one lav. As well, if you make a custom snake/cable for a Sound Devices mixer (Trew Audio / Remote Audio does this), it will have a headphone jack (not XLR).
1b- If you are going wireless in and out of the camera (from your mixer[s]), 2 X mini-plug will still be ok?
2- Another situation would be using double system, where you might just record 2 tracks of guide audio, plus timecode. In this case 2 independent headphones feeds should still be fine.
3- Menu-wise, you'd need controls for:
*Monitoring/mixing. 4 channels of inputs, and 4 channels of monitoring (L+R on headphone out 1, L+R on headphone out 2) would mean a 4x4 matrix. A joystick-like control like on the DVX100 would be a sensible way of setting this if using a menu system.
*Line/mic on each input.
*Limiters on inputs 1 & 2. (I don't forsee needing limiters on inputs 3&4.)
*Phantom power (Probably ok to omit T power.)
I think with this many settings (24 or more), you'd need a menu system. Ideally, the menu system would be so intuitive that you can look at the camera and figure out how to setup the audio.
4- Stereo linking of the limiters can likely be omitted.
My interpretation is that it implies that it IS possible.Flexibility about what channels to monitor implies that a 4 -> 2 channel monitor downmix is not possible?