Is it possible to use Visual OpenType Designer to create glyph combinations with multiple diacritics stacked above and below a base character? Some examples from Greek extended to illustrate what I mean: ᾄ ᾝ.
Sure, but I suggest you take part of the FontCreator beta team, as you will then get access to the upcoming FontCreator version with more support for
OpenType layout features. Do let me know if you want to join, and I’ll add you to the group!