If you have a have a glyph within uni1D100-1D1FF (Musical Symbols), when generating features, Glyphs adds the Languagesystems feature:
languagesystem musi dflt;
Should that be “musc” as shown in the OpenType spec?
http://www.microsoft.com/typography/otspec/scripttags.htm
The feature is generated even when they’re not set to export. Is that what you want?
Glyphs 1.3.19b14a (483)