Whedon fans are a nerdy sort, and certainly if the source video links can't be unearthed and the files downloaded, it can always be captured with software, and then distributed on file-sharing networks. Of course, truefans will make noises about then the artist won't be getting paid, but then there weren't exactly enough truefans to save Firefly, and that didn't even have dance numbers.
The point is that any attempt at creating scarcity will ultimately fail online. When the DVD is released with the musical commentary, it'll get uploaded and distributed online as well. This is not necessarily bad news for Whedon and the production, however — the unauthorized copies will likely be downloaded by fans who missed the online tease who are waiting to buy the DVD, or people just checking it out to see if they're really ready and willing to spend money on showtunes.
It raises the question then, why not leave the video up and sell advertising against it, like the creators of South Park have done? Because riding into town with a distribution twist garners more free publicity.