I'm not sure if I'm misunderstanding you (if so, entirely my fault, I've had a long day and not been sleeping much!) but the meta description for each forum is already the standard forum description. If you take a look at the source for the Suggestion forum for example, you'll see:
HTML:
<meta name="description" content="If you would like to suggest a feature that can stand alone or is an entire add-on itself, please post it here. Small adjustments can be posted here." />
And if you hover over that forum description you'll see: