If I understand you correctly, then you’d like to designate a mix that acts like the “Main” mix for the surface? So that deselecting other mixes would select the surface’s preferred mix?
I don’t think that would work well unless you could actually create multiple “Main” mixes in the Mix Engine setup and assign one to the surface (like you can with PAFL busses). The channels still need a main fader, and if your broadcast mix is say, a stereo aux, then selecting that aux would give you the sends for that aux – you would lose access to the channel fader, which would not work well if a channel needs to feed subgroups or FX sends.
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