Probably the most straightforward way is with a custom BlendMode. You draw it once normally, then a second time with a BlendMode which discards the RGB information. Have a look at BlendMode.as in Starling’s source. It registers the default blend mode ("normal") and a number of other common ones. I think the "erase" one would do it. Draw it with e.g. alpha = 0.5, transformed and sorted accordingly, and I think it would produce something like you want.
Yes, you have to draw everything twice but I don’t see how you avoid doing so if you have complex animations.