I've read your post several times, Phoenix, and I'm still not clear what you're asking. (Maybe I'm just dense.)
What is it you're trying to save time doing?
It sounds like you might be talking about rear projection -- an old movie technique in which a movie is projected from behind on a semi-transparent sheet so that the person in front of it seems to be standing on a street in Paris, for instance, instead of in a movie studio.
Is that it?
If so, that's possible but it's also a process that even Hollywood has long abandoned, for a number of reasons. Green screening and C Chroma Key are much simpler and, with today's technology, so much more effective.