Hi
If I understand you correctly.
You will have to have a alpha mask for your avi, and preferably a Z depth mask as well.
Then you need to use one of the emitters in P Illusion as both backdrop and foreground.
That is one behind the avi and one or more on top both using the alpha and not using the alpha to make it work.
ps. Making one volume(solid or not) go trough another volume is always going to take some work. That’s why you need to plan your shots ahead of time, and how you want to solve it.
T