Using a transparent image, of course, works, but is just a bit to cumbersome. In practice I tend to use two levels - i.e. a new "same level" with an alternative view. On the primary level I put door frames and all the fixed furniture - a bare room if you like. On the misc-other-stuff level I put doors and other stuff that will clutter my visualisation of the room.
Or - I sometimes to get a visual impression use a semi-transparent texture. Can be good for planning.
