>attending as undercover trainees

This is a good way to piss off the local senseis, I think. Nobody likes being caught unaware, and likely once people found out it was undercover you are unlikely to get their cooperation, and only foster bitterness, with the ending being "If they had only asked, I would have been happy to participate".

Go the opposite route: send out letters, "We will be in your area", get 4 or 5 to confirm, film at each place for a few days, and after a couple of weeks you have the 4-5 episodes worth.