I don't see how that's possible with this API though. Or rather, how the API has any influence on it.
See, if the host were to hash the pages I gave it (from the guest) with this API, and memcmp() them, it could with very little effort tell me it's stored it even though it just made a note that my ID(s) point to that particular page, and refcounted the page. This is easy since a page, once pushed from the guest to the host, is immutable.
The way the article was written though it seems that with the shared stuff, different hosts could access the same page frame by the same ID. So guest A could push out a page and guest B could retrieve it with that same ID? Where's the use in that?