Going to subscribe to this by posting as interested in the answer. Just some thoughts/feedback...
I wonder could you gain more control by limiting the PM to when they are offline and/or not in (present in/active in/using) the actual group. Just with loads of posts, while you're in your group and then you see your inbox messages increasing - not ideal really.
In GJ configuration there is notification tab "notify by" e.g. PM and under groups -> messages, "type" can be PM. But in my testing I never got a private message, about wall activity for example. And that's with the user manually setting various bits to "notify" him/her within the group settings cog - up by the top of the group page.
Edit: sorry you're looking at something else after re-reading your post. The manual action you refer to was what I was referring to I think.
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