I would imagine that "that" would be against the ToS due to the requirement to modification of game files (image files associated with the loading screen). The loading screen is directly associated with the World of Warcraft application, not the User Interface.
To elaborate on that, mods are loaded during that loading screen. You can't use a mod if it's not loaded yet.
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"You'd be surprised how many people violate this simple principle every day of their lives and try to fit square pegs into round holes, ignoring the clear reality that Things Are As They Are." -Benjamin Hoff, The Tao of Pooh