Exciting news to have Siamang back at TWPZ! From the video, it looks like she is in the previous Siamang exhibit, which was holding the White-handed gibbon pair on my visit in January 2024. Hopefully they are still on display and have moved to another island - I can't recall if their previous home was empty or housing surplus lemurs or spider monkeys, the latter being easily accommodated elsewhere.
The video on Facebook seemed to imply it might be a litte while before a male is acquired. This makes me hopeful there could be an overseas import, given that - as you've mentioned, all the males in the region are related to Kasarna.
I think one of the young males at Willowbank would be ideal. Sure, there's a relationship (first cousins once removed if I'm figuring that out right), but the inbreeding coefficient is low enough that it could still be considered as a breeding pair. It would be ideal for Ketut and Kasem to enter breeding situations, as their father, Bujang, is a wild-born founder.