Can a contextual alternate perform a transformation on an existing glyph? Or can it only perform a substitution for another glyph?
I am not sure if I understand you correctly, but with chained contextual positioning you can apply a positioning rule to a glyph.
Can I use calt to scale the glyph also? Or do I need to create a dedicated glyph at the desired size?
Scaling is not part of OpenType layout features, so yes, you need to provide another glyph,
Ok, thanks. One more question: I understand how to create a new ligature in the “Add Glyph” dialog: x_y.liga=glyphA+glyphB. But is there a way to type the code directly in for a contextual alternate?