I think it would be necessary for Jay to specify just what he means by mixed genre films. Personally I doubt if I would classify films like "Night Mail", "Listen to Britain" or even "North Sea" (although this last film definitely is a case of crossover.) The British documentary movement did stretch the Griersonian concept of "creative treatment of reality" to its outer limits, but this was partly because the technology of the time did not permit the real life portrayal that nevertheless was the ideal. These films introduced "re-enactment" as a subsitute for "the real thing", but considered themselves, and ought to be considered as belonging to the genre of "documentary". I have earlier suggested Humphrey Jennings "The Silent Village" and would add his "Fires Were Started" - both these films emanated from the British documetary but represent a break with the "objective subjectivism" of the Griersonian school. The break consists in making no excuses for using fiction. While the reenactment parts of the three films mentioned above are "hidden", Jennings proudly proclaims in his films that these are staged events based on facts. (A third Jennings film of the same kind leaps to mind: "The True Story of Lili Marleen"). In a way we can say Jennings films are early example of "reflexive films" (speculative? yes! but definitely worth a discussion.) However I do have another pre-war case of the mixed genre film - this time from the fiction side. Vsevolod Pudovkins "Chess Fever" where the great Soviet director used shots of the chess world champion Jose Capablanca visiting Moscow, and used this documentary footage as a clou in a film about a chess-mad young man.