No need for that. Every time the ad plays it is the same bitstream so you need to mark it once (for every resolution and format). So we essentially we will have signatures for ads similar like anti-virus software does for malware
Youtube will then start re-encoding ads per stream. We will have ads similar to polymorphic viruses :-)
that won't scale at youtube scale, the next logical step would be Widewine drm
If I was an advertiser, I would pay people to mark random parts of every video to pollute the data and render it useless.
They will just have AI generate unique ads for each person even though they are all slightly different.
Already done for skipping the sponsorship sections in videos.