HomePlug specifications are now publicly available.
Below are download links.
You will need to acknowlege the document license and warranty disclaimer prior to the start of the download.
HomePlug AV version 1.1 Specification (8.85 MB)
HomePlug AV version 2.1 Specification (11.4 MB)
HomePlug Green PHY version 1.1.1 Specification (10.7 MB)