Yes -- I wanted the beam to react to walkbehinds the same way that the player it was following did, and since no one else has brought up this problem, I thought that I might have overlooked something in the manual.
I never thought of the idea of using a flashlight "character" to follow the player around before. I tried making one, but it doesn't seem to work very well -- when the character goes behind a walk-behind, the flashlight character goes behind it as well, making the walkbehind suddenly much brighter than the space behind it. The mask also disappears when the icon bar pops up (probably due to the Z-order, I'm guessing).
However, I managed to come up with a workaround using the GUI method: I got the GUI's beam sprite to change to a completely dark mask when the character steps on a region positioned behind a walkbehind, then back to the standard beam when he steps off of the region. Since the walkbehind is a large, solid wall that hides the entire character, this works fairly well.
I'm now running into some slight problems with the flashlight not "turning on" when I start the game until the room is fully loaded, but I should hopefully be able to find a workaround for that too.
Thank you for your help, monkey_05_06 and Khris!