Tried a small test modding and its unfruitful.
Identified the assetbundle, which supposedly contains a script for the opening sequence and 3 starting CG. Made some changes with UABEA. Nothing changed in-game.
Deleted an assetbundle from the game folder entirely. Nothing, game loads the sequence without any problems (I can understand loading CGs because there are duplicates in respective character events, but who knows where it gets script for the sequence).
And that's just simple "replace existing stuff" modding. Can't even imagine how to get to system level scripting (or if it is even exists in similar to Gold form).