This has been patched out in the beta, because it was considered an exploit. Once you 100% someone they'll unconditionally surrender and accept peace only from the war leader.After getting 100% give them all occupations, they will make separate peace and after then you can hurt Ottomans even more.
To answer the question too, you could vassalize Byzantium while they are small-ish, feed them back to health so they can stand up for themselves, put your dynasty on their throne through the vassal interaction and then release them. This way the PU RNG will be stacked in your favour at least a bit, because you won't have to get their dynasty or wait for yours to spread to them.
- 3