The silly thing is that nobody seems to realize that the public has only seen 1 leaked copy, probably from 1 source. HBO could be using any number of techniques and none of us would notice any of them because we have a sample size of 1. Even the odd techniques OP describes could be present in the copy, your described technique as well, any number really. We won't see them unless they are as in-your-face as the watermark.
