CoreText has an XML file that is used to configure its fall back search for fonts.
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreText.framework/Resources/DefaultFontFallbacks.plist
It enumerates by postscript name, the phycical fonts used for sans-serif, serif, and monospaced.
It also enumerates the "default" font. There are a few variations inside the lists based on CJK language
to select a JA font in a JA Locale etc.
We could use the descriptions of sans-serif, serif and monospaced "default" lists for our equivalent fonts,
and use "default" as the fall back list for all other fonts, including "System".