Ooh, this is way different that what I was thinking of.

First off, why do you need to composite the images in postprocessing? 
There might be an easier way to do whatever you have in mind.

Otherwise, I'm thinking you could render the object you want "invisible" 
with:

texture {
pigment {color rgb 0}
}

So it just disappears into the background. (If you render with a 
different background color, just use the same color as the background, 
and use ambient 1.) Then, in your image editing program, grab the "magic 
wand" tool, select the background area, and delete it. (The exact steps 
vary depending on what software you use.)

Basically, instead of rendering to an image with an alpha channel, just 
cut out the background manually in Photoshop or whatever.

I can't think of a good way to do it entirely in POV.

-- 
William Tracy
afi### [at] gmailcom wtr### [at] calpolyedu

You know you've been raytracing too long when you decide to render a 
bird ... creating each feather separately.
     -- Quietly Watching

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