I just had a "Eureka!" moment. In the third chapter of the authorized sequel to The Prisoner, the graphic novel Shattered Visage, there is a funeral. And among the mourners, clear as day, are...
... Napoleon Solo and Illya Kuryakin of The Man From U.N.C.L.E. and John Steed and Emma Peel of The Avengers, all appropriately aged (as the tale takes place approximately twenty years after the events in The Prisoner). I must go through the frames carefully and see who else is there, as the novel is dense with insider references. But it's official: Prisoner/U.N.C.L.E and Prisoner/Avengers crossovers are canon!
(The Secret Agent/Prisoner crossover was already established as canon, if you consider the fact that the official contemporary spin-off novels for The Prisoner identified Number Six as John Drake.)
That is all. Be seeing you.