I remember FS was doing some work with that one, and managed to make some parts to scale better, but some others simply refused to accept any changes to their size, so even though it is possible to edit the .gui file, it is possible that the GUI itself refuses to change its appearance. No guarantees, though, but it certainly seems like some parts are hardcoded, no matter how ridiculous it may sound.
Unless you've proved it otherwise by testing the size-changes for all of the screens in-game, Dastardly? I think you're the last one who's worked with it...