Hi,
not directly related to MIDIbox FM, but I'm currently writing a MIDI synth based on OPL3 for Android (6.+).
It's a WIP (alpha pre-release), but already working.
It's a dual OPL3 emu, with 12 4ops for instruments and 12 2ops for drums, you can grab it here
https://play.google.com/apps/testing/net.volcanomobile.opl3midisynth
You can test it by pluging an USB MIDI Controller, your computeur, or with this application
https://play.google.com/store/apps/details?id=com.mobileer.midikeyboard
Next step is to implement patch edition via CC and UI on screen. I'd love your feedback.
Thanks