Good job, martiuzz!
I've made a somewhat poorer job at reproducing the non necro handler/leader issue with undead sieges. This save
http://dffd.bay12games.com/file.php?id=13475 is made at the very end of a season, and I did get a siege the next season, killed DF reloaded, got a new siege, killed DF and failed to get a siege for 24 attempts, but did get one at 25. They all had different handlers, and the two handlers I looked at first appear during the current year.
I think it's about as close you can get to reliably replicate the issue without somehow explicitly triggering the necro decision to invade (which probably would be a better option, if the decision point can be found and influenced. This save might be useful for that purpose, though).
I've used DFHack to move a necro tower to a single tile island and have embarked right next to it. I've also tried to piss them off by raiding them to steal a book and got seen doing it. Still, it requires a lot of attempts to actually get a siege.
Edit: Seasonal saves turned out to be no good for locating this issue, since while they seemed to reliably get a siege, the leader/handler already existed as a hist fig in the save: they just hadn't arrived yet.