Noel, am I right in thinking that the section the coupler fits into is actually part of the coach underframe and not part of the coupler?
In that case, if I were you I'd be tempted to go the whole hog and cut that section from the bogie then attach it to the underside of the coach. Should then be easy enough to fettle an opening to fit a draft box into. An undershot, long shank Kadee (no. 147) would fit perfectly. It looks like it would sit far enough back to give close coupling and the long shank should give it plenty of room to swing on curves.