Flufikas I agree that users need some way to contact the forum staff privately. The devs are already looking into ways of providing for this. Please refer to flarum/core959 for details.
Flufikas This is different from a private message feature, which i have read is in the works.
In fact it sounds very much like the planned Private Discussions feature. This feature, which can be found in esoTalk, is different from private messaging as seen in other forum scripts. The person who creates a private discussion can select one or more users and/or groups who will be able to see and participate in the discussion.
There will probably also be a "Private" link in the sidebar (similar to the Following link you can see there now) that will allow you to display a list of the private conversations you're participating in. This should eliminate the need for a special tag of the sort you suggest. (We'd want to avoid involving tags, as that would make the private conversation functionality dependent on the Tags extension.)
I'm just guessing about these details based on how esoTalk does things, but I'd be surprised if these guesses were very much off the mark.