We are in 2018 and there is still no official support it seems for pure MIDI AUv3 effects in iOS
It is reported by some developers that AUv3 MIDI plugins coded correctly as to 'aumu' spec but using the 'aumi' designator get rejected by the App store - even though they work.
Currently Garageband doesnt support 'aumi' so there is no way for any AUv3 host to currently tell a pure MIDI EFFECT from an AUv3 instrument that can receive and send MIDI.
When is this situation going to change ? This was first discussed in 2016.