Textpattern does not offer an obvious way to add meta tags to a template, but its easy to implement a custom description and set of keywords for each article.

Excerpt:

Writing custom meta tags for each article is extremely important for search engine optimization, but Textpattern does not offer an easy “meta tag” that can be thrown into a template … Textpattern RC3 is being constantly updated by Dean and company, and as of revision 309, TXP allows you to directly output the contents of the custom fields mentioned above. To use them for meta, follow these easy steps.

Note: in order to make use of this method, you will either need to turn off comments for the section of the article in question or turn off comment auto-append (admin > preferences > Automatically append comments to articles?).