aaargh always the endless waiting (so it feels) 
Maybe someone should port NFS Daemon to MSX. 
To be honnest, I've not seen such program for serial interface, MIDI, joystic port or any MSX hardware (Ok, there is one for 2xMSX, but not for PC), so I see no reason, how that kind of program could now appear for network interface. I think, it is much harder to implement this on real network.
NFS is probably a bit far fetched, but a Samba server and client would work miracles already and AFAIK it not very hard to implement.
/me is not volunteering tho
Sonic: Emulation code doesn't write itself, you know. 
Anyway, emulating an ethernet card has one difficulty beyond the usual hardware emulation: to be usable, you'd want to connect the emulated Obsonet cart to an existing network. I see two options to make that connection.
You could add a virtual network card in the PC OS: this interface forwards all packets from the MSX to the PC and vice versa. The problem with this is that you'd have to write a different driver for each OS and users may not like having to make such big changes in their PC OS just for MSX emulation. In addition, this virtual network card is only usable if the routing is configured correctly, which is probably too hard for most users, who just rely on DHCP doing all the work.
Another option would be to build NAT into the emulator. This works well for outgoing connections, but can be problematic for incoming connections. Also it's a lot of additional functionality to implement. Still, I think this is the better of the two options, because it doesn't require drivers or network reconfiguration to work.
Conclusion: unless someone decides to accept this task and keeps working on it until it's done, don't expect it to appear in an MSX emulator any time soon.