PAVP (Protected Audio-Video Path, which has gone by other names), as described in the FAQ entry, is a real thing, albeit without any public documentation whatsoever other than acknowledging that it exists - implemented in ME and used by some commercial video DRM implementations. Not sure what kind of citation you want, but it's briefly mentioned in this presentation about reverse engineering ME:

https://www.slideshare.net/mobile/codeblue_jp/igor-skochinsk...