It doesn't even need a watermark. Simply a tag in the data stream. Yes, I know a clueful user could strip this out, but most people don't know enough about data streams to properly remove such tags, especially if there is a checksum or some other feature that needs to be recalculated. Such tags are commonly passed on when used in most editors, so it isn't even a new feature.
This isn't time consuming at all. YouTube and other similar channels commonly scan for tags as well for other kinds of meta data, so adding a simple if clause that flags the video as lacking proper licensing is enough to kick it out. YouTube in particular does processing of all videos uploaded into its own proprietary data format for internal storage and does other kinds of processing like scanning for copyrighted content. This is literally trivial in comparison.