I've been wondering if it is possible to define a substitution rule for a letterpair consisting of two letters from different scripts, say, Latin and Greek? From what I tried, it seems impossible, I created some classes containing glyphs from mixed scripts and I defined lookups for them which I included in features for both scripts, and it didn't work.
I don't like the idea of scripts and language systems in OpenType scripting, I've always thought it was only limiting the designer. The only useful purpose of this system seems to be the localization feature. Am I wrong?