Flag Object to be Invisible to Alpha

[MrFurious]

Wondering if there's a way to flag an object to be ignored by Alpha. Would be tremendously useful for say, a sky polygon which covers the entire backdrop of my scene but at the moment renders white in the alpha channel. Would like this to be ignored my the alpha channel so everything renders white except the sky polygon. At this stage my workaround is to hide the sky backdrop poly and re-render.

[gristle]

Ignored, or be able to specify the alpha colour like you can in LW