If your aggregator lacks iso-8859-15.enc....
..., then this bug report was made for you.
(I finally fixed some standards compliance issues with this blog. This exercise included correctly tagging all content here as iso-8859-15 -- after all, I'm occasionally writing ¤ signs. As a result, mt-rssfeed stopped being able to read my own RSS feeds. Generating iso-8859-15.enc according to the instructions above and copying it to the appropriate place on the system fixed that problem.)