You can't use coremods with beta clients, a new datablob is downloaded every time you start the game.
You can try starting the game with the additional -localdataonly in the shortcut, though I believe it was disabled some time ago.
Keep in mind that the datablob you get while playing beta versions, aren't always the same as the xml you have in your install folder. So doing something like this may cause unforeseen issues.
