The Private Topics plugin uses a regular category with an additional setting that only the Op and a defined group can see.
If they are posting the character sheet publically. Then accessing it in private might not be needed.
A link to a topic here is only visible say to your player (Op) and the Gamemaster group.
The signature plugin would just simplify having a link to the character sheet in their private topic. It would also serve if the GM needed to give info that they other players would not be aware of.
Then having a link back to public game session topic.