Well, you could perhaps cheese the system by creating additional empire titles:
- Don't be gavelkind
- Create an empire title
- See if you can change the laws in your original empire. (If not, create another empire.)
- Destroy all non-primary empire titles.
This will obviously cost piety, gold and vassal opinion - and you'll probably have to put down a civil war or two as a result - but it's probably worthwhile for the long-term gain.
(I'm assuming you're an empire because you have 340 voting vassals. If you're somehow still a king, you could do this with kingdoms instead.)