However I still wanted users to get neat avatars next to their comments, and the easiest way I found to do this was to add support for the Gravatar user-pic service and Favicons to my comment-listings. Adding them was very straightforward as there are ready made Movable Type plugins for both, except of course that neither of the Gravatar-plugins worked. *Sigh*
After some research it turned out that the Gravatar-supplied plugin is outdated and also only supports Movable Type versions pre-3.0, and the MT supplied plugin only supports MT 4.0 or later. Combine this gap in supported versions with Murphy's Law, and it is just obvious that I still had to be using MT 3.3. I did upgrade to MT 4.x once, but it caused so many problems for me so didn't keep it for long, and attempting that upgrade again was not an option right now as I just don't have the time to get it working properly.
That leaves the option of hacking the plugin! Yay! With no plugin-documentation to be found and me never having toyed with MT-plugins before, the task naturally stumped me a bit at first. However I figured out soon enough how to get it working, so for anyone else having the same problem I hereby present you with the updated Movable Type 3.3 Gravatar plugin! Enjoy :-)
And now all that remains is for my readers to get their own Gravatars. Go fetch!