I am having the exact same problem.  As a matter of fact, the scene I am trying to get to render before the Holiday contest ends is the current issue.  I cannot render the scene in it's entirety.  I decided finally to seperate it into 2 or three seperate renders.  To start I am doing a render of just objects which are outside the of the rooms window.  Next I will do the inside objects and walls.  Lastly I will render the actual figures.  Then I can just layer them one on the other.  I will use the outside scene render for a backdrop when I render the room itself, then I will render it and use that render as a backdrop for the figures.  I will have to do some messing around with the shadow and liminance map, but it should work out.

Quote - In Cararra, is there a way to save a foreground image as a layer which can then be superimposed over a background image?

If I get too much detail, too many objects, into a scene it becomes unwieldy.  Rendering the background and then the foreground seems a good workaround.

A jpg with no background saves with a black background.  The true transparency of a layer is needed for a realistic overlay.

Klebnor