[charlesb]"How would you go about compositing this alpha channel in photoshop?
Could I remove the black?
Or to the opposite:
If I was to put an image behind this alpha, would it be possible to see it while retaining the shadow information?"
I'm not sure what the goal is, but if you just want to recreate the shadow you had in your 3D program in Photoshop then you could make a selection from the alpha channel (ctrl or cmd-click it) and use Layer > Layer Mask > Reveal Selection on a new layer. Then fill that layer with whatever color your shadow should be (black probably?).
I hope that's what you meant by compositing. An alpha channel can be used for effects as well (such as using one as a height map in Lighting Effects or to control a Lens Blur). Let me know if there's a more specific effect you're trying for.