I started work taking a look at J2, was trying to find the right way to implement it using OOJS on the new JS structure.
It's more important to finish H-8 "Add (hide | delete | suppress) in board history...", it's an old late deliverable and getting it to work will validate the restructuring. I mentioned in a comment on the OOJS patch https://gerrit.wikimedia.org/r/#/c/165756/ that it seems to incorporate incomplete work for board history; ideally H8 would be a separate patch based on OOJS, like all new JS work.