There are at least a few plugins out there designed to better connect Textpattern powered sites to Technorati.
The ones I’ve seen so far use the keyword field or a custom field to store the tags in a comma or space delimited list.
This one just uses categories, or multiple custom fields if you need more than two tags on an article. It’s all extremely simplistic, and my first public plugin. My first public foray into PHP, for that matter.