Well! Looks like I accidently found a way around the problem. If the meta-ball group is a relatively uniform distance from the camera, the distance mask will give you an image which can be massaged (in Gimp or Photoshop) to produce a object mask. Now everyone who reads this will know.

Didn't you know that beauty is in the eye of the beholder. That's why eyedrops and rose-colored glasses are needed.