I seem to have found a working solution, gleaned from adwaitapod.sbs. HUGE thanks to evan Dook!
Here's what I'm using:
# if we're in the Settings chooser and if the list title is Brightness, do A, otherwise do B
%?and(%if(%cs, =, 12),%if(%Lt, =, %Sx(Brightness)))<A|B>
If anyone has any corrections, improvements, or other comments, I'm all ears.