I take it you're considering those mounting options for your tandem because it lacks rack braze-ons on the seat stays...Is that the case?

If so, have you considered p-clamps like these? Of course, if the frame is carbon like the seatpost, this would be out of the question...

Why not just change out the stokers seatpost to an aluminum one to allow for your seatpost-clamp rack that you like?