In Movable Type it is possible to generate a feed for entries matching a given tag only. For this to happen, just select some tag, e.g. from tags cloud and take a look at link named “Feed of results tagged …”. For example, for this blog’s Perl tag, the link looks like:
and you can use this URL as a feed, e.g. for Ironman challenge.
There is a problem with Ubuntu package, however (Karmic at least). Packaged MT is missing
search_templates link in
/usr/lib/cgi-bin/movabletype. I reported this bug, and you can apply the simple solution on your own.