Adding an object buffer to caps (C1/C2) of extrude NURBS

I have a rectangle spline with an extrude NURB on it, which is then added to a cloner object. I want to add object buffers to the end caps of the extrusions so I can use them as screens in after effects. I can't make the extrude objects editable as I am animating properties of the extrude NURBS (extrusion depth etc).

I can add a material to the caps of the extrude object by add a material to the extrude NURB, going to the 'selection' field on the material and entering C1 for the bottom cap, or C2 for the top cap.

Is there any way I can make an object buffer reference these selection options?

If not, does anyone have any ideas as to the best way to comp video footage onto the end caps of multiple (cloned) extrude NURB objects?

Thanks guys!
Andy

other than putting a luminant material on the caps (pick a color) so you can do chroma map, no