That does work and I have done that before.
What I forgot to mention in my original post is that I am trying to get all of my multi-pass renders done in a single render.
If I leave the floor on and hit compositing background, then I have to do two renders in order get all of the proper elements in place for my compositing program.
In other words, I have to render the entier scene once for shadows, with the floor on, then render the whole thing over again, with the floor off and just my 3D objects in the shot.
The real problem I am dealing with is that I am using the objects in my scene as ridged bodies and the dynamics engin is giving me slightly different results every time I am a render, so it is critical that I render everything at once.