Ruskul - you might need to put the shadow light on a transparent layer with 'force own texture' set for inverting alpha to work, otherwise it will be rendering to an opaque surface and the alpha will never be different.
As for detecting objects in shadows: how do you actually define if an object is in a shadow? There are a variety of ways to do it: is the hotspot in the umbra, is the entire collision polygon in the umbra, is the entire bounding quad in the umbra, etc - and then what about penumbras? If it ever so slightly edges in to a very wide penumbra that probably shouldn't count as suddenly visible, but on the other hand an object just inside the light edge of a very wide penumbra should probably count as visible. I don't know how to accurately define that. The line-of-sight behavior can already make visibility checks, and it only looks at origins but you can make it do multiple checks around the edges of objects to come up with your own solution, so I don't think there needs to be a shadow-specific feature for that.