Ethernet is a flexible protocol with collision detection and avoidance You should be able to piggy back all the netduinos and use Ethernet normally, provided you statically set their MAC and IPs.
I would suggest using UDP for communication as there are less packets ( No delivery confirmation like ACK ), so the Ethernet wont be so busy, preventing other devices the ability to communicate.
However if you want you could write your own tiny protocol on top of Ethernet II. Less overhead the better.
Edit: Just thought, another option for preventing collision ( When two devices try to talk at the same time )
You could allocate time slots, perhaps based on a unique ID. So each device can put a token on the network at a certain time. The possibilities are endless really.
Let me know what you decide, I'll help where I can, but I currently do not own a network enabled netduino.
