@JeffryBooher Cocoa allows decoupling window title from the "represented filename". The problem I am facing is how to pass another string. I though current approach made sense as it was compatible with Mac conventions and did not require any new shell JS APIs.
Another API-less (well, it's quazy-API) is to have page title on Mac in format "${filePath} ${someSeparator} ${pageTitle}" - then native code will split the string in two and use them as needed.

Other then that, new shell JS API is needed. I though it would be Mac specific then (though you are saying it is also on Windows8.