But that would mean that you need to have access to the video on the canvas/JavaScript level. I’m unsure if that is possible with EME as the browser does not have access to the raw video data. Also, if it worked, I think they would break their DRM in that case. And once it is broken and brought into another format (HTML, because users won’t accept anything else) or canvas (which is very user-hostile and I can’t imagine any publication having success with that approach), then the DRM is already circumvented, and changes by the user seems to be fair game to me. I am obviously not a lawyer 
I think it’s going to be too expensive and fragile to be viable but that’s what a lot of widespread js-heavy practices looked like at the beginning. And the ads can still be blocked at the add-to-the-DOM stage anyway without touching anything legally protected, although that’s an order of magnitude more work for the ad-blocker maintainers so probably won’t be done.
To an adblocker the resulting DOM is what counts. They can scan for changes and see if it is ad code. Content blockers in Safari, for example, do that automatically and it is fairly simple to create new rules. Apps like 1blocker crowdsource it and are very effective.
