How would that work? They dynamically make a new version each time someone wants to download it from the "official source"? Would this be the same version over the entire day? The entire week? What if I wanted to download an old episode of Seinfeld, would I get current ads, or ads from 1996? Would the "official source" still have to be able to upload each episode? If not, you run the risk of having multiple versions of current shows (this week's ads and last week's ads) in the wild. And you lose the ability to have people who downloaded it last week help with distribution.
I think that the best solution is to have a container that automatically inserts ads at the appropriate times. In this case though, the task will be making sure someone doesn't just remove the ads from the container. This is a similar problem with DVRs, but since this would all be automated, there is a greater risk.
Yes, the idea is to insert ads dynamically when you download them. The issue is that the specific ads parts must be not distributed on third party places, they are only in servers related to the ad system.
With Bittorrent some parts of the file can change and still people can seed the unchanged parts. So having multiple versions in the wild is less of a problem.
I think that the best solution is to have a container that automatically inserts ads at the appropriate times. In this case though, the task will be making sure someone doesn't just remove the ads from the container. This is a similar problem with DVRs, but since this would all be automated, there is a greater risk.