The back of the board can carry electrical current so the only way of doing so is using a cooler that the plate is non-electrically conductive. Next issue is the CPU mounting plate will not sit flat so there would be very little heat transfer making the whole thing useless to do with out a custom plate. So unless you are going to design a back plate to accommodate both coolers and a custom mounting plate that is non-conductive it can not be done.
Some cases have fan mounts for behind the CPU to help cool that area.