Is there a practical way to broadcast and receive ascii payloads over ethernet or wifi? How best might arduino nodes pass incoming serial ascii messages out as ascii network broadcasts for all others to receive and pass out through their serial ports? 

ie: typing a msg from the IDE serial monitor of any ethernet node should cause that msg to be output to the serial monitors of all other nodes.
The magical trick you are looking for is Ethernet UDP Broadcast. By chance there is an example sketch in the Arduino Tutorial. I cannot say if it is up to date but this will help you get started. 

Cheers!