The official Apple API docs were too sparse for me. I also couldn't find any working examples of Messages plugins that were more than very simple demos calling simple HTTP services.
Use Adium instead. Adium might not be as pretty as Messages, but - until Apple updates their documentation - I am not going to continue working on this.
Thanks to the people who have emailed me with interest in this plugin. I really intended on getting it working. I just don't have time to guess at how to make it happen.