I think the part of an OS which is most central in defining its identity is not the kernel but the libc. In fact, GNU/Hurd may end up being binary-compatible with GNU/Linux because both use the same (GNU) libc. Isn't that the most important thing, i.e. what name you look for when you have a list of OS choices for a binary package? I suggest the term GNU/ELF
We don't speak of GNU/BSD because BSD has its own ls, cp &al, programs. The GNU programs go in
Also, I don't think ``what you can't change without rebooting'' is a good definition of ``central''.