I think you're confusing "extenders" with "Network Media Players" (NMP), they have a lot of things in common, but they are not the same things. Most of the boxes you see floating around are NMPs, they're boxes that allow you to play content from locations on your home network (and sometimes the internet) or local storage/connected hard drives. These are stand alone devices, they don't work or integrate with anything else, they just do their own thing.
Extenders are similar (SageTV ones in particular because they can act as just simple NMPs), in that they play content off the network, but beyond that they integrate with a server for access to a central database, be it of metadata, show info, EPG, etc. They "extend" the functionality present on the PC (EPG, Metadata, watched status, watched position, etc) to the TV via a "box".
I haven't really followed XBMC or many of the NMPs, but I expect the Seagate you're talking about (GoFlexTV or FreeAgent Theater+) are simply Network Media Players, not Extenders.
The only two systems I'm aware of with official Extenders are Windows Media Center (Xbox 360), and SageTV (HD Theater). It's possible XBMC has some sort of replacement firmware or something for some/certain NMPs but like I said I haven't followed them so I'm not sure.