All Zs bar the 6000 are USB clients, the 6000 can be either a client or a host.
TCP/IP over USB works for all Zs, but it's a host-client connection with the PC as the host.
I was wondering whether it'd be possible to make the client hardware provide at least some limited host capabilities, but I'm not that familiar with the USB spec and hardware. So it's on the backburner for now.
Si