Happy to do that, but I want to see if I can make the process easier first. Building the bluez stack from source was really hairy. I had to hack up the configure script that autoconf generated, to skip a couple tests and hardcode some paths to libraries ... that kind of thing shouldn't be necessary, but I couldn't figure out a way around it.
I gather that Debian has an official way to backport a package, but I ran into problems with that some point. If you have any special knowledge of how to backport the bluez v5.50 package from Buster to Stretch, that would be really empowering for BLE MIDI on Bela. (Bela currently runs bluez v5.43, but MIDI support was only added in v5.44 ...)
The end result of all that hacking was to build a replacement for the stock bluetoothd, that detects BLE MIDI devices. I can certainly post that binary, if there's an appropriate place for it. I don't think any other part of the bluez package needs to be modified/upgraded, besides that one daemon.
But after that, I still am unsure of the best way to get a Bela program to choose that particular MIDI device. I posted that question in another thread. =)