Thanks for the feedback @join !!
Love the "Dot for Don't Care" idea. I'll run a test case shortly ... One thing that Aodyo does in the Sylphyo User Manual is to choose the most appropriate symbols - open or closed - for the Don't Care locations based on what a player would usually use. Maybe there are two Don't Care symbols one for "This looks Closed but really it's Don't Care" and "This looks Open but really it's Don't Care" ... maybe a small white dot on a big black circle and then a small black dot inside a white circle for those two symbols. I may be overcomplicating this, and it would be complex to explain to users ... most of whom just want to get on with things ...
A 512-character block with all 2^9 combinations of open and closed is straightforward ... excellent idea! I can probably do this programmatically. I would map it out literally like the bit patterns starting from all holes closed at (say) code point U+EB00 and ending with all open at U+ECFF.
FF Chartwell uses Discretionary Ligatures, a feature I've tended to avoid for font features that are central to what users need. I'm generally concerned with complexity and implementation of the features in apps (including older versions that many people tend to use).
However, I have done something similar without any OpenType layout features at all. I could create a block of Composite Characters. Each Composite character has a piece of a finger diagram - the outline, the open hole symbol in each of the 9 locations, the closed hole symbol in 9 locations, and etc for don't care, trill, half-hole - a total of around 50 characters. All the Composite characters would all be set to "Zero Advance Width" ... the cursor would not move after typing each of these characters, so you simply build up your image by typing 9-10 characters over each other. You then hit the space bar to advance to the next character. Laborious, but at least you can get any finger diagram you want. In practice, this scheme works "Pretty Well" - most applications seem to render them correctly. I have had cases where PDF viewers have micro-errors in positioning, so a composite character comes out a bit "fat".
This Composite Character scheme could be added to the fonts in a small 64 character block without too much hassle. It would take a fair bit in the User Guide to document it ...
Another alternative would be to use Stylistic Sets. This could allow us to map the QWERTY keyboard onto the finger diagram glyphs. OpenType allows 20 Stylistic Sets to be specified. Each stylistic set could be used for a different instrument (Flute, Sax, etc.). A stylistic set could also be used for those Composite glyphs described above. So a finger diagram could be accessible at a code point (e.g. U+ED01) and also by choosing Stylistic Set (eg. SS01) and typing a "1".
The Sylphyo fonts actually are OpenType fonts - technically "OpenType with TrueType Outlines", but the term TrueType is more familiar, so I tend to use that colloquially.
Thanks again for the feedback!!