Apparently this "forty minute long documentary will give you ... a glimpse of the past, present, and future of Freeskiing"
I'm sure it's got lots of great info about the history of freeskiing as that's how it was pitched, and it would seem disingenuous for the result to be anything otherwise.
In all seriousness though, I think one of the things about freeskiing and its history is that it's all kind of informal. There isn't one unifying encyclopaedia on it, or like a "history of freeskiing" laid out all in one place.
I'd try to find some old copies of Powder or FREEZE, or anything like that you can get your hands on to see the progression in real time. Some of that would be online somewhere. Also Ethan Stone wrote a pretty banger piece on freeskiing in the NS magazine last year. Idk if that's online, but it's definitely worth reading and would be helpful for you I think. Also BroBomb's Know Your Roots series has a lot of good info although it's pretty informal (see:http://brobomb.com/2010/04/know-your-roots-parkasaurus/#more-1520
) - also the site might be fucked right now. That's a whole other story.