it's not gonna be easy.  was also problem with early carrara versions and GI - no shadow-catching. in P8 it also appears there's no shadow-catching on ground plane when using IDL (GI), nor when using IBL/AO on lite, nor AO node on ground.  shadow-catching works with directional lites, but I only know how to export the figure together with the ground plane shadow, not the shadow separately from the figure.  can run renderpasses for each lite, but figure will always be visible AFAIK.