Theory is an object buffer render is supposed to be that however I noticed in C4D that they often don’t properly align, the important thing is that the MV alpha be not outside the RGB image. Workaround is place the object buffer in the MV alpha and choke like 1.5 (to play with, you will see when enough there won’t be artefacts).
However if the RGBA color pass does not have transparency (full alpha except for anti aliased edge) then you can in a subcomp place the color and mv source, turn off the color source and take the MV pass and with set channels grab the alpha from the color source (of course MV pass and AE project must be 16 bits)
and then drop that comp as MV source in new comp
Pierre