I’m not sure how I would tackle this in a final product, but here are some ideas that I had while thinking about it… they aren’t perfect solutions, but may be worth exploring if you can’t find the proper filter. I looked at all the filters as well, and can’t see any that may suit the situation.
However, you may be able to explore the select2_init here.
1) You could switch to a select, and dynamically populate it, settings some items to disabled (if they have children).
2) You could use AJAX in the footer to query the same Posts you are presenting in your Post Object, and then loop through them, and dynamically disable items.
Hope that helps get you moving along. Let me know how you went about it!
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