Even if this is "homebrewed" in China somewhere and not officially Sony-made, if this works, holy crap!!
PS2 NETWORK ADAPTOR FOR SATA HDD (China Trading Company) - Other Computer Accessories - Computer Accessories Products - DIYTrade China
Any info on this??!
EDIT, ok something very interesting that I noticed; notice how in this guide you need to modify the back of the LAN only NA: Search for Kablunz guide.
It seems like in the ones on the "diytrade" site - which would be appropriately named if these are custom done jobs - don't have a drilled out back. Interesting.