What are you using this spanner for, by the way?
I'll use it for various spanners that need gradual changes, primarily for trills with variable speed. There are other more text-based notations but I like the simplicity of the spanner.
I am surprised by the font issue though. Is LilyPond preventing you from using a font because it lacks some accidentals even if you are not using these accidentals?
I can't remember the exact issue but I'm using microtonal accidentals for some projects anyways.
Nothing wrong with the Feta font by the way. I just have an illogical preference for SCORE's look so I'm using Scorlatti plus various tweaks to approximate SCORE's slurs and ties. That sometimes causes me a lot of trouble but at the end I get nice results.
I will try to share more details about what doesn't work as well as I'd like but my use case is very specific and non standard so I feel bad asking for help since some tweaks go pretty deep and complex.