Glyphs can generate mark and mkmk features from the anchor list in the XML, but it might be helpful if it could generate others depending on the script. Is it possible for you to modify the feature generator to spit out abvm and blwm in certain scripts (mostly Indic)?
If it is possible to do so but you need a list of scripts, I’m happy to do a research on it.
Do you want the list of the script which need abvm/blwm instead of mark, or have it already? At the moment I want support for Tibetan and Oriya, but that sounds random and I think I need to research all scripts that use abvm and blwm exclusively (the ones don’t use mark feature).
Okay, just got word from Georg. The features are auto-generated on generation. Explains it. Still not entirely understanding how the mark-to-base and mark-to-mark GPOS are derived.