Another feature that you may want to add support for is the FONT_PATTERN define. Basically, it lets you specify what the font path given a specific directory. The default is of course "*;common", but you could also set it to "*;voice" if you prefer. Or "*;common;voice" if you feel like you need two common directories. Or "override;*;common" or "override;*;extras/*;*/Proffie;common".
There are functions in strfun.h that converts back and forth between a single directory, like "TeensySF" and the full font path, which in the last example above would be "override;TeensySF;extras/TeensySF;TeensySF/Proffie;common".
One possible use case for this is to have font-specific voice packs. You could for instance do: "*;*/voice;common". You would need to create a “voice” directory in every font directory, and then you can (optionally) stick a voice pack in there to override the files in “common”.