I haven't actually received my coda yet, but in the videos I noticed that at the underside there are some ridges.
That made me think of a neat idea;
Perphaps it could be possible to add a separate small vessel with 5 holes on the side of the coda, increasing the range to 3 octaves?
It would pretty much be a one handed ocarina that you could attach on the right side of the coda via the bottom ridges, and whilst playing you can switch your hand and mouth to this attached vessel for extra range. Pretty much how it would work on standard triple ocarina.
The same thing could be done for more low notes on the left side, essentially creating a bass!
This idea of a modular flute sounds pretty cool to me, and I think it's something straight up your alley Karl!
I know you lose ability the play the entire instrument for 1 set of holes, but the extra notes are probably rarely used and just there for those songs that are just above the coda's range.