Something like GDG would have been anathema to the original file system creators of Unix - at least through the Berkeley FFS days. I think I saw something like it on some extensions in 4.1ES. It was certainly on VMS. Maybe something like ext3cow will do what you are thinking of. Of course much of the problem is that Unix at the mothership was used extensively to submit JCL to OS/360 hosts, so all that stuff was done back on the big iron, not the cheap Unix FEs. Departments had to cough up for the unix hosts but projects paid for time on big iron. I never worked on projects that cared about that - network design not legacy OSS dev.