The good news is that it's clear, I understand it and the issue 559 you mentioned would perfectly answer to my needs (much more than that in fact). The bad news is that it is not implemented yet, so I would probably need to do something (probably simpler) myself if I want it.
Is it possible to participate to the Lilypond project? What are the technical requirements?
A small detail: >Sounds more like chromatic/unisonoric accordion to me. For diatonic, I have rarely seem anything but "Griffschrift":
There also beautiful scores for diatonic accordion (or whatever instrument able to play this type of music actually) not only "Griffschrift".