Serial bus Raspberry Pi to multiple AVRs
There are standard RS422/RS485 transceiver ICs available

Here you'' find many pages of RS485 related ICs at Digikey

You can use standard asynchronous serial data using RS485 if desired. "Rolling your own" multistation code should not be too hard [tm], but it's an oft invented wheel - there will be many people already offering software (much for free) that does this.

The CAN bus lives on top of RS485, but is probably overkill for what you want.



Superb application note

AN-218 Implementing a RS-485 Multi-drop Network

This is targeted at ZBASIC but is liable to be of direct relevance to your application.


This application note describes how to interface ZX devices with a RS-485 multi-drop network. RS-485 networks
are used by many industrial applications including process control, building automation and even the control of
lights at a concert. A RS-485 header board and example hookup is provided to explain the simple hardware
required to get started with networking ZX devices. The software included with this application note gives a sample
application and a reusable set of APIs to send and receive messages on a RS-485 network. Here is a table of
contents to help with document navigation.




Here you find ...   

RS485 Multidrop Wiring Diagram &amp; RS232 - RS485 comparison

They say - 


RS485 is sometimes termed as RS485 Multidrop LAN since it can connect several devices in a LAN network environment. These devices are all connected to a single pair wire. Transmit and receive share the same two wires.
Officially the RS485 specification allows only 32 nodes (devices) on the LAN. However, I.C. manufacturers have developed RS485 drivers capable of allowing 128 to 255 nodes on an RS485 LAN. We use these next generation RS485 drivers in our products. This means that you can use our Converters and Remote I/O devices in more expansive situations.






Useful tutorial 
Wikipedia - RS485

Lots of ideas here from G's image search