==Copyrighted Work Enclosed - Not for re-release ANYWHERE==
According to technorati docs, from what I can tell, adding <category> around keywords in the rss will get threads indexed by tags.
So, I have put together a crude addon to an
existing mod that adds extra fields to threads.
First, I installed the mod above. Then, I added a new text field called Tags to my threads. Then, I looked in the database and saw that this field was added to the thread as 'field5'. (yours may vary.)
Then, I modified my external.php file to grab that field5 and explode the string into an array (commas were my separator of choice.)
Finally, I added <category> tags to the various xml formats. (I didn't bother with the javascript format as that is not used for the feeds.)
Note: I have RSS discovery code in my header include as well.
I think this mod could be better if I plug in to the forum search and enable users to search by tags as well. If I get time, I'll hack that in.