Your idea to put a point light inside a transmapped object should work okay. Here are screenshots of a ball prop textured with a spots node in the transparency channel (just to act as a quick transmap). The ball is at the centre of the Poser stage with the point light inside - there's no other light in the scene. The three boxes are at zTrans -1, -2, and -3 respectively. The point light is red and has its Dist End set to 3 to get the light falloff, otherwise the light would illuminate everything equally.

The first image is the preview and you can see that the light does illuminate the boxes. Image 2 shows that the falloff works. The problem is that the light cannot illuminate its own exterior because the rays will never hit it, so the ball looks black. To fix that, you can set some red ambient light on the ball material.

Steve