Flash :: Render Just A BitmapFilter To BMD?

How would you render a BitmapFilter (such as DropShadowFilter) applied to a sprite, to a seperate BitmapData Layer? which would then be placed beneath all objects.For instance if you have two rectangular sprites overlapping each other, with a DropShadowFilter applied to each, the result would be the filter overlaying and obscuring the other rectangular sprite - as shown below.

note: each filter may have different settings and so I cant just apply a uniform filter to a containing sprite. also: ideally this solution will be scalable to handle many animated sprites at the same time.