The point really is changing a MPQ file by either adding/updating/changing/deleting files in it is just a bad idea.
NOW if there is a way to change that screen by dropping a new graphic into the DATA folder now that is a different story. No MPQ modification there, but I don't think it would work like it does with audio.
I do know there is a addon here that makes the current login screen black (
Black login Screen .. currently broken :P ) and silent and as far as I know no one has been in trouble for using it.