The key is to make the meta descriptions unique - if they are not important then the rule which grabs the first sentance would work fine (as you suggest).
Another rule (so it reads better) is to limit it to say, 135 chars, have some smarts in place to ensure you don't end the sentance mid word & then append the end with "... Read more here" or something relevant to your site.
So the template could look like:
"<first 135="" chars="" of="" first sentence not="" ending="" mid-word="">... Read more here."
(even if it does end mid word it would be better than having duplicated descs since the pages aren't important) </first>
hope that helps!