Very sad news, it's very unfortunate that Cassie will be the sole male in the group. I don't imagine it would be very easy to introduce more males to the group.
With the age and history of Cassie, I would also highly doubt that they will ever breed as Rockhampton hoped he would. When he dies, it might be a good idea to send one of the Taronga males up to form a breeding group, or just send the females to Taronga and put something like mandrill/colobus, etc. in the exhibit.