Pasting Over a Portion of A Matrix?

Hi - I've read tutorial 14 and understand excerpting a piece of a matrix and placing it in a user defined position (very cool, having lots of fun mapping this to step #/velocity from drum sequencers), but what if I wanted to place it over a pre-existing matrix? For example, I have video A running and it takes up an entire matrix. I then want video B to be excerpted and placed in random locations, with random sizes ON TOP of video A. Note - I do not want to the matrices added together, I want video B to entirely replace video A's values at the given matrix location, but have video A remain through out the rest of the matrix.

Thanks for any help. Let me know if you need clarification.