Using roles and capabilities, it should be possible to allow Guests to participate in forums, just as it is possible to allow them to attempt quizzes and other activities. However after allowing the appropriate capabilities (start new discussions, reply to posts, etc.) Guest is still given the message "Sorry, guests are not allowed to post."

I checked mod/post.php and saw that this behavior (along with lots of other behavior) is based on a test of the legacy capability moodle/legacy:guest. This code needs to be updated to use the new forum capabilities.