I would not know a lot about this, but I have gotten the impression that Vimeo.com seems to have quite a lot of long-length videos on Buddhisms, in the region of maybe 30 minutes to well over an hour in length.
Take for example this one:http://vimeo.com/16766725
I'm not sure that it is what is called a documentary, but I have a feeling you might be interested in what he has to say.
About 6 minutes into the video, he makes the observation that there is a lot of wrong information going around about the Dharma, because what is happening is that people who know a lot about Dharma don't tend to speak about it much, but people who know very little about Dharma talk a lot about it.
You probably know about Youtube, but maybe Vimeo isn't so well-known, so i thought I'd suggest that as an avenue for you to explore.
Hello, I am not actually a Buddhist, and I know only some very limited basics about Buddhism. I'd like to know a bit more and to ask a few questions, if that's OK.