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.