I fixed the problem. Solution - push the matrix, enable blend, bind the texture and draw the textured rect., disable blend, then finally pop the matrix.
TIP - You can simply modify the GUI's by extending them and overriding their methods (then set the GUI field in the GUIOpenEvent to your GUI) rather than copying the whole GUI class and calling displayGuiScreen() like shown above, unless you need access to private methods / fields and don't want to use reflection;