Warriors get Bonuses on Hammers with the 1h and 2h Mastery, but I think it's more about the Loss of Bleeds than any thing else. Not to mention I have yet to see a Hammer that could compete with an Axe in DPS.
It would make sense to give Knights a Mastery on Hammers because no one else is using them. I tried to use hammers and found that they just don't stack up to 1h swords. They might as well remove them from the game because they really have no point to them any more.
I did see a few Dream Splitters (2h Hammer) used back in Ch3 but that's not the case now.