Andrew Kramer has a technique for generating shadows for Trapcode Particular.
Watch his awesome tutorial here.
Here's a little test I did while working out how to create shadows for Particular layers.
It uses a duplicate of the main comp to create a "Shadow version" of it, with the camera in the shadow comp linked via expressions to the light source in the master comp. I'll post the AEP on my site if anyone wants to play with it - at http://www.digitaldistortion.net/rsrc_particularShadow.html