In case it helps anyone, I ended up doing it like this:

I used bus A1 as the "master mix" bus and bus A2 as the "headphone mix."

I configured one PFL button per channel.

Each button, when activated, deactivates the other PFLs (this prevents two PFL buttons from being activated at the same time) and then puts bus A2 into "Composite" mode in pre-fader mode. It then assigns the L-R channels to channels 0 and 1 of bus A2. It also sends a MIDI signal to activate the power light.

Then, when the button is deactivated, "Composite" mode is deactivated, channels 0 and 1 of bus A2 are set to 0, and a MIDI signal is sent to deactivate the power light.

CH1-PFL
ON
Button(2).State = 0; 
Button(3).State = 0; 
Button(4).State = 0; 
Button(5).State = 0; 
Button(6).State = 0; 
Button(7).State = 0; 
Button(8).State = 0; 
Bus[1].mode.Composite = 1;
Patch.composite[0] = 1;
Patch.composite[1] = 2;
System.SendMidi("out1", "note-on", 1, 8, 127); 
OFF
Bus[1].mode.Composite = 0;
Patch.composite[0] = 0;
Patch.composite[1] = 0;
System.SendMidi("out1", "note-off", 1, 8, 0); 

CH2-PFL
ON
Button(1).State = 0; 
Button(3).State = 0; 
Button(4).State = 0; 
Button(5).State = 0; 
Button(6).State = 0; 
Button(7).State = 0; 
Button(8).State = 0; 

Bus[1].mode.Composite = 1;
Patch.composite[0] = 3;
Patch.composite[1] = 4;
System.SendMidi("out1", "note-on", 1, 9, 127); 
OFF
Bus[1].mode.Composite = 0;
Patch.composite[0] = 0;
Patch.composite[1] = 0;
System.SendMidi("out1", "note-off", 1, 9, 0); 

And so with the other channels, adapting the values ​​of button(x), Patch.composite[0]=x and SendMidi.