There are multiple techniques that can achieve this effect. One of my most used tricks is to place 3D layers in the right position to approximate the real world items that are catching the shadows. You use multiply to add the shadows to the bg plate.
If you can post a screenshot and a description of the action I'll give you some more detailed instructions.
Thanks for your reply. Given the nature of the material, I can't really post it here--don't worry, it's not NSFW! If you don't mind, I can PM you a couple links to what I have been doing with other shots in the project, and the current state of the shot that needs the reflection. Just let me know--thanks!