I’ve looked into this briefly before, here are my findings:
The type should be ‘MIDIPlugin’, but for some reason, parsing it was disabled by this commit (line 885), can’t really think of why so. Also a lot of plugins must be updated with the correct category, as they tend to be in ‘Utility’ category, so it seems like some effort scattered around many places is necessary to make ‘MIDI Utility’ category to work.