I notice that some postboxes (not as many as are tagged royal_cypher) have a royal_cypher:wikidata tag and the preset in Vespucci has a field to enter it. Is this simply a duplicate of royal_cypher and does anyone use it?
Taginfo finds only something that appears to be part of JOSM, and Vespucci (which you already mentioned).
I wonder whether any tools from the wikidata end could handle something like blah:wikidata in OSM?