I had set up a few meta-balls in a scene with nothing else. I rendered the group and all was well in the land of Bryce. HOWEVER, when I asked for a object mask of the selected meta-balls, all I got was a black picture - no mask, just black. So I tried an distance mask and it worked. Why won't meta-balls render a object mask? PS. needed the mask for translating the background of the image into an alpha channel. I was able to massage the distance mask and get an object mask from it.

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