Glyphs does support them, but there is no UI. Add them as specified by the AFDKO spec:
cvParameters {
FeatUILabelNameID {
name 3 1 0x0409 "Alternative small a"; # English US
name 1 0 0 "Alternative small a"; # Mac English
};
FeatUITooltipTextNameID {
name 3 1 0x0409 "The small a has many forms to offer."; # English US
name 1 0 0 "The small a has many forms to offer."; # Mac English
};
SampleTextNameID {
name 3 1 0x0409 "alpha all axx"; # English US
name 1 0 0 "alpha all axx"; # Mac English
};
ParamUILabelNameID {
name 3 1 0x0409 "Single story"; # English US
name 1 0 0 "Single story"; # Mac English
};
ParamUILabelNameID {
name 3 1 0x0409 "Alpha form"; # English US
name 1 0 0 "Alpha form"; # Mac English
};
Character 0x0061;
};