multipass render help

I want to render my animated scene with only 1 object visible - where other objects pass over/infront of it I want a gap. - ie. Imagine cube A is the only thing i want to be visible in my render, cube B passes across cube A and I want the shape of cube B to block part of cube A (leaving a hole/gap in A) even though cube B is invisible.

I guess another way of describing whAt I need is - a way of rendering an alpha channel of how a group of objects(i want to be invisible) affect(block from view) a visible object.

I hope this doesnt sound stupid but its hard to decribe without being too long winded.
I have been trying all kinds of multipass render settings and compositing tags but cant get it right.