Almost anything is allowed, provided it does not move while skiing and is considered safe by the Safety Director.
Devices affixed to the ski intended to control or adjust the skiing characteristics of the ski are allowed if they are fixed so that they do not move or change during actual skiing.
Skis must be safe. There must be no unnecessary sharp or abrasive (to the touch) metal, wood or other attachments to the ski which could, in the opinion of the Safety Director, inflict injury to the skier should he come in contact with the ski in a fall.