How to implement nut fraction

I am investigating on how to add nut fraction functionality to a font, using @tiro_j’s Nutso implementation:

Looking at the .feat file of Nutso2 which I want to use, it looks like I need to make contextual mark feature (the mark position changes depending on the number of numerators/denominators). Is it possible to add mark feature manually in Glyphs? Or is it even the right approach?

That is possible. You add an mark feature and put # Automatic Code End at the top of the feature. That will put everything you write after the automatic feature code that Glyphs will generate from the anchors.