I’m creating anchors to attach the rhotichookmod to various letters via mark positioning. But what I want to do is also modify the resulting width of the composed glyphs. I saw in another thread the ‘secret’ of naming a glyph beginning with double underscore, to add that width to the glyph. It seems kind of a hack, but I don’t see any other way to accomplish this. Thoughts?

I think this is a similar case to capital Greek Polytonic diacritics. The way I did was to make a ‘letter’ version of them so that positioning is purely done via kerning. Currently there is not much you can do with spaced diacritics by themselves as I understand.