Took a bit of work, but I finally got it to work with Paintball2. The fact that it requires the "nodraw" flag to be set in order to render the shadow, and the fact that paintball2 actually USES the nodraw flag, so if you set it, it disappears, makes it rather difficult.
It's also a challenge to get the lighting conditions such that it can even be noticed (ie: not too much reflected light).