I'd think you would have to look at it like writing a book, except with the chapters being in video instead of text. I think trying to approach it as a series of RailsCasts would not cater to the type of people who want such a structured learning system. One of RailsCasts best benefits is that you learn on your own terms, which is the opposite of what you are going for it sounds like.
There are many different learning styles and some people like to see rather than read so I think it has potential. You may check out LearnVisualStudio.net as they have been selling videos for years, including videos that go from nothing to building a full featured application.
Good luck in your venture - the more ways people can learn rails, the more it will attract new developers and grow the ecosystem.