Can't you just fiddle this into ubb itselve?
Copy the newtopic code+template, change it to look like a guestbook entry form, allow anonymous posts and voila. You have your guestbook as forumdata.
You could simply build something around it to display it.
You'd have to hide that forum somehow, If you have a subforum that is the child of a forum you don't have access too, it is not showed in the main forum overview.
