The sysprocesses.net_addres column stores the MAC address of the connection. It should be NULL for system processes (possibly for local connections, but I haven't tested that). I don't know of any way to spoof a MAC address, but at the same time, I don't really know how to search for a computer on my network if I only have a MAC address, so it does little good for me in either case.

I'll go back and update my original post to add this information in.