You could go ahead and drill perpendicular to the face where the tenon runs through so all that is left is the wood running to the angle. Then you could simply chisel the remainder. I would recommend transferring that "interior" angle to the faces of the post for alignment ease. If you clean out the cut part of the mortise just mentioned I guess you could take a hand rip saw to the sides and cut to the angle as well. That might make the chiseling easier
