the very latest version of AUM now - in the last few days ha been updated to support MIDI output only devices with kAudioUnitType_MIDIProcessor.
And Bram Bos (Ruismaker) and Jonathan Lihljedal at Kymatica (AUM) have found and are working on several ways ( that don’t break apple API guidelines ) to get actual non audio MIDI Filters ( or MIDI Effects ) that use kAudioUnitType_MIDIProcessor working.
Currently Garageband wouldn’t doesnt support filters but its highly likely that AUM will have something out - and they plan to let others know how they did it ( its not any bodge ).
And imo - its much more likely that the kind of user who really wants to use MIDI Effect/Filter apps is going to be the kind of iOS musician who is using AUM rather than GarageBand. Because such filters arguably have more uses in “live-jamming” experimental
situations than linear DAW-type composition. AUM is pretty much a de-rigeur must-have purchase - similar to how everyone on PCs or Macs might have MS Word
my own prototype using kAudioUnitType_MIDIProcessor now appears in AUM - whereas it didnt a few weeks ago. I’d still really appreciate having an iOS11 version of the Arpeggiator there though.
Maybe someone at the JUCE Team might like to liaise with Kymatica and Bram Bos and find out how they got it working.