Hi Keith,
It’s about a year and a half ago I brought up this request. All three solutions you suggested are doable but.
Using Post Fade Aux is certainly working, but occasionally I need all aux for different purposes (mainly EIM). So that is not a permanent solution for me.
The second suggestion to use 2 matrices both fed from LR (with all processing switched out) in order to record flat and only use mix processing for the room is fine too. I can source one matrix pre fader for recording (independent of the mainLR level), and the other matrix fed post fader to adjust the level of the PA using the mainLR fader or the matrix fader. My main objection is that I am wasting resources ie. I need an extra matrix. If I have the choice to source a matrix channel pre or post-processing block of the main LR channel that is more straightforward what I want to achieve in my opinion. In short I still like to have the choice to source a matrix channel pre or post processing block of the main L R channel.
Cheers,
Tom