I'm not directly familiar with ZK, but I just reviewed its architecture a bit and I don't see any reason why it wouldn't work great with Broadleaf Commerce. Take a look at this page:
http://docs.zkoss.org/wiki/Hibernate_%2B_Spring_%2B_ZKThe Spring/Hibernate portion of this doc is pretty much in line with how we do Spring and Hibernate in Broadleaf Commerce. You should be able to reference any of our Broadleaf services (Cart, Account, Catalog, etc...) in a way that's consistent with the ZK methodology using the ZK ServiceLocator.