Hey @Holger_Löbel,
have you tried the reporting feature already?
It is a premium feature that allows you to create graphs and reports by pulling data from other projects: Reporting with Asana • Asana Product Guide
Now if you want to view all these tasks that have a special tag/custom field in one timeline/calendar view I suggest to add a 2nd project to these tasks.
For example you could call it “Communication campaign main dashboard” or something like that.
Then every task you need would be added to this dashboard and you have a detailed overview. So you basically multi-home every task.
Now to start this the best way is probably to use advanced search to filter all the tasks that have this special tag or custom field entry and then highlight all and add the new project to all the tasks in one go.
Let me know in case that does not help 
P.S. there is also an open thread in the feedback section: Timeline - view multiple projects together