Sergi Miral released allthemidi, along with 4 other custom MIDI apps. The whole package is a bit confusing, and the website is useless. The App Store description for each of these says that they, "work in combination with Audio Unit apps that listen for MIDI." I think they're AUv3, but it does not explicitly state that they are themselves Audio Units. It looks like allthemidi includes all of the controllers available in the other offerings.
Design your own custom MIDI controllers and send MIDI messages to other apps & hardware.
This app works in combination with Audio Unit apps that listen for MIDI events. A great free app that allows sending MIDI messages to other apps & devices via Bluetooth or USB cable is Midimittr
The main MIDI properties for each component (channel, note, cc) as well as the customisation options (color, direction, text) can be changed using the controllers shown in the screen.
Inspecting the underlying data structure in json format will give you access to more tweaks. Check the existing templates to know more about some of the possibilities.
If you need support with your layouts or have any feedback or ideas to improve the app just go ahead and get in touch (firstname.lastname@example.org)