If I were you, I would write to the sr. director myself,

ask him to add me to the invitation, explaining my involvement of the project. The tone of the email will be friendly, since the goal is to work with the sr. director, not picking fight. I may CC my boss and other related party in the email, so they know I'm interested in attending. That's it. Just think how you would react if the sr. director were a friend.

When there is unhappy history between two parties, the one that can put aside personal feeling and handle the situation with professional manner will win everyone's ultimate respect and support, since you show the better character.

Also, too many times, our own attitude can make an enemy become a friend, or a friend become an enemy. And having a friend is always better than having a enemy.