There has been imported a number of Leafs from the US to Norway over the years and exactly this issue was one of the reasons for why I didn't buy a US-spec version when I bought my Leaf a few years back. With 3 kids, I had some concern over this.
Anyway, I looked into it then and found that the easiest way would be to swap the rear seat back over from a Euro-spec Leaf. Although I never did this myself, I do know of at least one or two owners of US-spec Leafs in Norway that did it this way to get a center headrest in their cars. I do however see that getting hold of a Euro-spec rear seat back may be a little bit difficult if you are based in the US...
The seat frames look similar so there might be a possibility for attaching a center head rest there, but I don't know for sure.
By the way, I don't know if it is important, but the center head rest on the Euro-spec Leafs has a different part number than the two others.