I'd thought about that, too, but I couldn't figure out how to go about it....
I figured you'd draw the whole screen in ZQuest as if it were "lit", but then somehow script most of it to be black...? I guess I could try drawing a black rectangle over the whole screen, then drawing the circle around Link. I'm not sure how the transparency would work...
*EDIT*
Maybe the solution is to draw a bunch of black circles outward starting some distance from Link?