I have been thinking recently about coupling for these wagons in 4mm scale.
Ironstone locomotives fitted with buckeyes often (granted, there were other arrangements) had then fitted underneath the draw hook, as per this MW at Corby:
https://c1.staticflickr.com/9/8376/8397266511_b8d365072c_z.jpg
The easiest solution would doubtless be to use Kadee system, but this would require a trade-off between correct prototypical height and model operation (as kadee uncoupling operation is, as I understand it, based on height over a magnet).
In an attempt to overcome this I'll be trialling these:
http://www.sergentengineering.com/
I'll post how I get on, but I'll have to build an appropriate loco first!
Paul A.