Assigning a variable based on fame number

Ok, this is an odd one and I'm having a tough time figuring a solution. I would appreciate any help on it.

My situation is that I have a series of photographs of things at different angles. In comp B, I have all of these images laid out in sequence. In my comp A, I have placed comp B with two expression slider controllers that allow me to display different frames from comp B. Pretty simple.

I also have comp C, which contains an animation. This comp needs to be superimposed over Comp B in comp A, and then corner pinned to specific coordinates so that it looks like it is in perspective. So when I select a frame from comp B using the slider controls, comp C would corner pin in perspective and play it's animation sequence. This corner pin data is unique to each of the images and cannot be derived via math. And I need comp C to do this automatically via an expression.

The way I have been attempting to do this was to place a guide layer in Comp B of Comp C, already corner pinned properly. I was then trying to write an expression in Comp A that would pull the corner pin data from that specific frame of comp C in comp B, and apply it to the Comp C in comp A. The problem is twofold: I don't know how to do that and I don't think it can be done.

So here are the questions:
- Can I pull variables from the fame being displayed of a different comp? (unlikely)
or
- How can I set this up so that I can associate a unique set of corner pin data to a specific frame of a displayed composition? ie: If comp B is showing frame 18, the corner pin data for a second object would be (x,y),(x,y),(x,y),(x,y)?