The Always Anonymous Categories plugin does one half of what you’re trying to accomplish.
This plugin lets you choose individual categories for “always-anonymous” behavior, where all posts and replies automatically use the anonymous pseudonym for the user instead of their actual identity. The user doesn’t have to opt-in, and more importantly, doesn’t need to remember to opt-out when they are done posting.
The other half should be doable in a second plugin, but AFAIK that does not exist yet.