Hi Catherine,
to second what Brendan said, you really should contact the members of Hyoho Niten Ichi-ryu. Iwami Sensei, the current head of the school, is Musashi's eleventh-generation successor, and you are unlikely to find anyone as knowledge about Musashi and his life than the modern heir of his sword school. You might even be able to film some of their sword training-- they still practice the actual techniques and training drills that Musashi himself developed. The footage that I've seen of Hyoho Niten Ichi-ryu in the past ranks as some of the most interesting traditional swordwork that I've seen.
Good luck with your documentary; I'll look forward to viewing it one day.
David Sims
"Cuius testiculos habes, habeas cardia et cerebellum." - Terry Pratchet
My opinion is, in all likelihood, worth exactly what you are paying for it.