Despite no assistance from Technorati, I finally appear to have Technorati tags working. The trick, I think, was modifying the syndication feeds to add a category tag for each keyword, like so:

<MTKeyWordList>
<category><$MTKeyWord$></category>
</MTKeyWordList>

The idea, and the plugin that made it possible, came from this entry on Laughing Meme: MTKeywordList and Technorati Tags. That entry links to the plugin, a very simple piece of Perl code, which creates the MTKeywordList container tag. The above code, added to my RSS 2.0 feed template, iterates over all the keywords for the entry and generates a category tag.

The equivalent code for the Atom feed template:

<MTKeyWordList>
<dc:subject><$MTKeyWord$></dc:subject>
</MTKeyWordList>

I had noticed that Technorati was treating the Movable Type category names as Technorati tags. The code above produces the same XML for each keyword that was already being produced for the names of each category associated with an entry. It appears that Technorati is scouring RSS feeds for <category> and <dc:subject>, but is completely ignoring hyperlinks with the rel=tag attribute, contrary to the documentation on Technorati tags.