Thank you for the pointer. Currently I am using 9 mark glyphs – {Small, Medium, Large}² – which serves me well.
Three glyphs are redundant (SM–MS, SL–LS, ML–LM) and could be simulated by horizontal kerning. Removing this redundancy would probably make the feature code more complex, but the exported files would be cleaner and smaller. I will look into it.
(All 9 glyphs use a single _part glyph, which makes managing them a joy.)