There are already discussions in the foum about sending Aux Mixes to LR Master which were answered with the problem that might bring in latency problems and this is unlikely to happen.
One possible solution for the task is to send both Main LR and the Aux Mix to a Matrix Bus which is then routed to Outputs. That way it is possible to combine even several Mixes without getting problematic Audio paths.
That works but beside losing a rare Matrix Bus (which may be acceptable) it has the caveat that the Main Mix Master Fader won’t do the intuitively supposed task to control the Output any more since there is an audio path around it.
Instead, you have to use the Matrix Master for most tasks you normally would do with the main Master Fader.
In that situation a way to assign the Matrix Master instead of the Main Mix Master to the rightmost Fader (the “Master” Fader) would make work more intuitive since that really controls the complete Output Signal there.