Request: Can we assign the Playback Masters on a Gio to be Something Else?

Running 2.4, there's a tantalizing menu popup in fader setup that implies that I could assign the playback fader pair to be something else.  (For example, on shows with no cue stack, I would love to make them be the Grand Master, rather than using a regular fader.)  But it doesn't "stick" if you change the setting.  

Could it?

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