Well that's exactly what Thorne did in his Raven moon prop. Here's a little detail from an image I'm working on. I think this is what you mean right? Do a search and you might find it still in FS or his site. All it is is a white outline of a raven on a square applied as a transmap to a small square parented in front of a spotlight. Snowsultan has made a couple of light sets that take advantage of this and includes two transmapped props, one with window blinds and the other ... I forget :) I'm almost sure Snowsultan's is still available since it was much more recent, in any case a simple search in FS will hopefully yield results.