Thanks to John Minkjan (obiee101.blogspot.com) I was able to disclose (and secure) non-OBIEE content on a dashboard.
Using the regular Upload functionality you can easily upload a (for instance) PDF file into the Shared Folders of the webcatalog.
The awkward thing is, that if you put a link on your dashboard and use the Browse button to navigate to the particular folder, you don't see the PDF file since it is not an OBIEE object.
A workaround for this is to create a (dummy) analysis and store it in the same folder as the PDF. Then, as described above, put a link on your dashboard and navigate to the dummy analysis. As a final step, (since you now have the path to the folder where the PDF resides) replace the name of the OBIEE analysis with the PDF's name.
There you go ! Now you can have a link on your dashboard to non-OBIEE objects and use your security model on it !