Hmm, there seems to be a bug:
when a marker only has one attribute defined in the style file, (like \rb) it’s treated by some parts of the code as if there were none. Until that bug is fixed and finished (work in progress), the work-around ought to be to put:
\Marker rb
#!\Attributes gloss ?junk
into a custom style sheet (allowing an optional attribute of junk, and informing the code that yes, actually there is some clever stuff for ruby).
Then you need to set the font / FontScale etc for the special marker gloss|rb e.g:
\Marker gloss|rb
\FontScale 0.8
\LineSpacing 0.6
\Color x7f0000
There’s another bug I’ve just found (and fixed), that it ignored LineSpacing defined for gloss|rb and only obeyed \LineSpacing set on rb (both should be an option, with it checking gloss|rb first, and if there’s no value set there, checking rb).