I found a quick fix that works for this and some other forums. Create a bookmark and paste the following in as the URL. The one bookmark will mark all threads as "read" for this and two other forums I am a member of. I would prefer to have the link back, but since this is a multi-use bookmark, I guess that makes it ok. It will work on the forum site that you are looking at, not all sites at once.
EDIT: Hhmmm, there's a space appearing between the words "mark" and "read" after I save. When you copy/paste, remove that space.