Well, I sort of tried it today.... 2 problems....
1. Using copy en_GB gets you 24 time, but gets you dd/mm/yy in displays.
2. The new locale doesn't show up in the "Systems Settings" menu. Don't know how to get that to work. I just used "export LC_TIME=" in my .bashrc to use the new locale.
I rarely use GNOME and don't have it installed on my F22 test systems.