I Think this paper(Modeling Coverage for Neural Machine Translation) have done a great job, and I really found the over-translation and under-translation in the translate results given by OpenNMT.
Thanks we will look into adding this. If you were interested in sending a PR we would be very thankful as well 
It’s hard for me to change OpenNMT with this complex module, I have some code in a issue(https://github.com/OpenNMT/OpenNMT/issues/133) works for coverage but it is just a simplest version with the samiliar idea, I’m testing whether it could work now.