Component pointing to other layer in same glyph

I seem to remember there was a way (through scripting?) to have a component point to another master of the same glyph.

Context: I have a slant axis in a font with Arabic and Latin, but the Arabic should stay the same (i.e. not slant). So I want to replicate the upright shapes in the slanted masters.

That will be available in the upcoming 3.2 version.