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Well, about 6 months later and we’ve gotten a lot of negative feedback from implementing the anonymous forum category this way 
Our members are just too used to being able to post anonymously in a forum category without having to worry about entering and exiting from anon mode. Most need to be guided thru the process, and many will just never try.
Just bumping this as a feature request. We would be willing to help cover the cost of development! As I see it now though, unless per-category anon posting becomes an officially supported plugin/feature, we are out of luck!
Thanks