It occurs to me that adding a visual watermark might actually serve to obscure a visual watermarking scheme that is otherwise invisible by providing data that scrambles or breaks the watermark decoder itself.

Audio watermarks can be distorted in any number of ways; and it could be that some of the wildly poor audio quality in most cam-rips is probably the only way you can defeat the watermark; by using a LQ microphone and encoding the audio to a very limited bitrate and then re-upsampling; to defeat any subtle alterations a digital watermark might make to the audio waveform.