Don, thank you for your guidlines. I finally found an extremely simple solution to enable anonymous posts:
I simply added the following line to /packages/dotlrn-forums/www/dotlrn-forums-admin-portlet.adp to make to existing links available:
Line 22
<li><a href="@url@"><span>#</span>dotlrn-forums.all_Forums#</a> (<a href="@url@/admin/"><span>#</span>dotlrn-forums.administer#</a>)</li>
Thus from the Admin-Page of the class both the lists of all forums available and the administration page for all forums are accessible. For both links I was not able to find a link to the corresponding page.
Now every class admin can access the permissions link for a forum and set create and write permissions for "The Public" to allow anonymous posts.
What do you think? Only dotlrn-forums-admin-portlet.adp got 2 new links and message keys. That's it! No major extension, no changes to forums-security-procs.tcl...