What you're looking at is the IOP bus, (aka ssbus I believe) which sends data to DVD+BOOT rom, CD/DVD controller, and HDD/LAN controller as you said. SD0-15 being data lines, and SA0-26 being ADRESS lines (although only up to SA22 seems to be used, and that is connected to a pin that is labeled NC). So if you want to interface with the IOP these lines would be where to do it.
I would recommend checking out this post if you haven't, it's about getting a hdd to work on the scph 7900x, which is not a problem since you have a 7000x, but it contains some interesting discussion about this bus.
http://psx-scene.com/forums/f98/hdpro-clone-132869/
Unfortunately all the attachments seem to be gone, but you should get the general idea by looking at the 7000x schematic