This config gets a bit buggy:

  COW:
    - action: RIGHT_CLICK
      tool: BUCKET
      replacetool: WATER_BUCKET
      message: Water from a cow!

Often the water will spill out to the ground (seems the rightclick event occurs after the bucket has been replaced) and if we add an event cancellation on the second line of onPlayerInteract we still sometimes get buckets of milk?

The PlayerInteractEvent has a method that's something like setUseItem(Result.DENY). Maybe that would fix the issue? We could rig up a fake Cancellable (much like what I did with EntityDeathEvent) which does that instead of setCancelled(true). I'm not certain that that wouldn't break something else, though.

Cool - I can try using setUseItem() in my test environment and see what happens - will update here when I've tested it.