Reporting on Prince Harry's official Jubilee duty visit to the Caribbean, the Telegraph mentioned that Her Majesty the Queen had granted special permission for him to wear her Diamond Jubilee Medal before the normal authorised date.
"Granted special permission" from such a source means "is your duty to", DIN or no DIN.
As with the Golden Jubilee Medal which Prince Harry also holds, Her Majesty has the clear prerogative to issue and authorise this medal irrespective of the military administrative system, to commemorate what is, after all chaps, Her Jubilee.