Oh, I got IRC to work! Downloaded the example code from Apple's developer site and compiled: https://developer.apple.com/library/archive/samplecode/IRCServicePl

Oh, I got IRC to work! Downloaded the example code from Apple's developer site and compiled: https://developer.apple.com/library/archive/samplecode/IRCServicePlugIn/Introduction/Intro.html#//apple_ref/doc/uid/DTS40011125 https://developer.apple.com/library/archive/samplecode/IRCSe... Then I had to run "killall imagent" in Terminal for it to work properly. Logged right into OFTC in Messages on OS X 10.9 Mavericks. So... plugins were supported, but it may well be that the only plugin ever made was literally the example Apple released? :(