I got tags working via a plugin.
Since I was messing with the site anyway, I hacked together some Ruby code to pull all the content out of the database and perform automatic keyword extraction via naïve bayesian analysis.
It spat out a file of SQL commands, consisting of the subject of each posting and the first line of text (in comments), followed by the commands to add the tags. I ran through the file in vim deleting here and adding there, then executed the result. So now pretty much everything should be tagged, right back to the start. How cool is that?