As I pointed out in another thread, you could have some sort of custom scripting done to build a sitemap based on threads in your database, you could then block all access to spiders to your forums and use the sitemap with its custom categories it can list building just the links you want...

An example would be (this is my private dev machine in house, also I looked up how most sites do their sitemaps and built a rough compatibility):
Google XML Sitemap
Link List
HTML List
Yahoo RSS List
ASP List

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