Halos and Z-rendering

Hi,

I am using Blender 2.48 to make a space ship, and I have some laser beams. The laser beams are rendered as halos in the materials tab, to make them laser-y. However. I want to composite it with the spaceship, and try as I might, I can’t seem to get a valid Z-pass off the layer. I also tried z-masking, but that doesn’t work either. Short version: How can I get a z-pass off a halo?