That would be a ground for an embargo as well (like the embargo laid down in the Will of Wilhelm II that he did not want to be interred in the Hohenzollern crypt as long as the monarchy was not restored in Prussia).
But in this case such an embargo on jewels is more or less useless and only 'punishing' the descendants which have no any influence on the desire for a restoration of the monarchy. Even in this hour of greatest (economical and social) worries for Greece, there is no call for a return to old times.
I think we will never know why it lasted 30 years after Queen Frederika's death to see this sparkling jewel again. Maybe it simply had to do with death duties which expire after a time? Who knows?