[dkoh]

The idea is to store in the "genres" attribute all the instruments used in track with the prefix "Instrement"

This would be better handled in it's own tag.  I don't know of a database that would supply this, and I'm not sure how you would handle some things, like a rap song that samples a trumpet.  This would probably be a manual affair.  It looks like ID3 does not contain an Instrument frame, but that doesn't mean you can't use one, and you can use almost anything in FLAC tags.  I use Jaikoz and there is an Instrument field.  The question is what can read it.  It would be nice if there were a way to enable nonessential tags in advancedsettings.xml or something.  That would open up use for custom fields that Kodi would pick up while leaving the standard tags to their intended purpose.