Would it be possible to port the React-Native MIDIVal code into a Capacitor plugin?
I could support this effort with some $$$ and/or time.
Creating Plugins Using the Scaffolding Tool
https://capacitorjs.com/docs/plugins/creating-plugins
Development Workflow
https://capacitorjs.com/docs/plugins/workflow
This is really only needed for iOS, since Android works already with WebMIDI.
It would be mostly a matter of defining the interfaces using the Capacitor way instead of the React-Native way and then calling the same basic code in the Swift file.