hmmm, let me think, what format is the animated output from the "render"?

if i understand what you're asking and if you can get the renders as sequental frames, you can open each one, open your background "still", merge them into one file and resave them... all done as an "action". (or maybe a couple of actions...) but you should be able to automate some if not all of the processes. there is also a thing called "droplets" that will let you drag a folder full of images and have the action run.

go ahead and post some images of what you have in mind and we'll try to create the steps needed to automate it.

see ya 'round the forums,
:)
scott