It looks as though the standard supports metadata at more than just the document level:
In general, any PDF stream or dictionary may have metadata attached to
it as long as the stream or dictionary represents an actual
information resource, as opposed to serving as an implementation
artifact. Some PDF constructs are considered implementational, and
hence may not have associated metadata.
Clear as mud! Thankfully there are some additional notes. Including:
In addition, metadata may also be associated with marked content
within a content stream. This association shall be created by
including an entry in the property list dictionary whose key shall be
Metadata and whose value shall be the metadata stream dictionary.
Because this construct refers to an object outside the content
stream, the property list is referred to indirectly as a named
resource (see 14.6.2, “Property Lists”).
This means that you can attach metadata to certain artifacts within your document but I don't believe that you can attach them to a specific page, you would have to have an object that you attached the data to - an image would be the obvious example though the standard seems to refer to shadings too.
Of course, although the standard seems to allow it, that doesn't mean that common PDF handling libraries and applications support it.
Adobe's downloadable version of the Standard (will save you CHF200)