Hi
I managed to solve the problem quite simply by rewriting the title directly in the header.php if I am on the forum page...
You can check detail explaination on my SimplePress forum, and check by the same occation it is workging pretty well:
http://bonplangratos.fr/forum/bon-plan-entre-geeks/yoast-wordpress-seo-how-to-display-title-of-your-simplepress-post
This should help your forum posts to be indexed by google...
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