Starting with version 13, Android has native MIDI capabilities. These are enabled in two steps (I think)- by compiling the kernel with the right flag set, and then enabling it with an entry in /system/etc/permissions. You can check to see if the first step was done (I think) by checking for system/lib64/libamidi.so. If that's there, but the permissions xml isn't, this Magisk trick will finish the job.
My knowledge of Android internals starts and ends with what you've already read, but I'd hazard a guess creating a Magisk module to do the same thing would work. All you'd have to do is create a system/lib64 directory in this module, add libamidi.so, and add "system/lib64" to config.sh around line 77- look at the example above it to see how to override multiple directories. The kicker is that libamidi.so is a dynamic linklib, so you'll need to compile it yourself or find a binary compiled for the same architecture as your device. Again, wild guessing here, for all I know trying this would brick your device and light you on fire.