It occurs to me that there's another dimension to this idea that I want to cover. It sounds like the methods discussed are good for finding and affecting target units. I am also interested in defining an area for each slice to play spell effects within that area, should it be desirable to do so. Thoughts on this?
EDIT: Earth-Fury's method touches on finding units in an area, which can be adapted to finding random points in an area for spell effects; however, is there a way to do this without first enumerating a lot of unnecessary units and then culling them out?