It could be done. You'll have to write a small search engine yourself, that matches the keywords and the surrounding words.
Then you'll be able to put the results into the meta-tags.
But i doubt that this will increase you ranking effeciently. Most spiders rate the hole content of the site and how many times the site is linked etc.
Meta-tags aren't that important anymore.
You can insert a meta-tags with "header-insert" in the admin section. Insert a list of general keywords that match to the general topic of the site.
The spiders will search through the body/content of the board on their own.