As far as I know, the answer is: NO ( exept in campaigns where the answer is yes). But hereÂ's a simple trick that might work in SP/MP-mode:
1) Make an empty plan, with a secret objective.
2) In the objective you put a link to another page in the briefing. The player will not be able to access that page until you decide to reveal the objective.
Voila!
I think this works, but I didnÂ't test it yet.