Yes and No ! Not Posible at the moment.... But theoretic posible with some changing in Mobiflight code i think.
1. Problem
Mobiflight not Supports Analog Inputs and so no Potentiometer at all.
2. Problem
I not fully understand this part of the Code in MF, but it looks like MF set the Brightness of MaxDisplays while first starting.
(Look into the Datasheed
https://www.sparkfun.com/datasheets/Components/General/COM-09622-MAX7219-MAX7221.pdf
Specialy Table 2 and Table 7 to understand how a Max7219 Brightness Control workes. )
So with current MF Code it´s not posible to change Brightness "live" cause it still send the data one times at Startup. (I think)
For Youre building it need a logic like "Scan the Value of the Potentiometer... If it change and reach XX then stop Display Update, send 1 time the new brigthness Command and then restart normal Data Upload to set the Display.
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Maby a compromise is posible....
Setting the Brightness not in Menu.... Set it via Direct Input ( But NOT with a potentiometer). Max got 16 steps of Brightness. Maby a function can be implied, the allow via Button to send a new Brightness Command to Max..... What command is sent can be defined by a readout Offset.
For Example....
A Button send on Push with a config a Value to a Offset (Custom). With a IF Function we can increase this value to all 16 Steps ( Or mabye less steps if we want)... like a toggle function.
On release of Button it starts this new function, that send the current Offset Value ( of this Custom Offset) "1 Time" to the Max Module and set the new brigthness.
So we can use a Push button ( Or a encoder ) to do that.
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So finaly.... I think it´s posible technical.... But need lot´s of new Code in MF and make the Programm difficult ! Thinking about "usefull and profitable" is needed.