433 mhz RF comms using external AVR MCU (advice on serial protocol needed)

Sounds good - do you think: AT+SEND1/AT+SEND2/AT+SEND4/AT+SEND8 might be more informative for the data size?

Great news about the PCB - you're connecting to the existing cheap RF transmitters and receivers? It'll be a great little tool.

Just thinking, now you have the CTS/RTS pins as well, you could maybe have a mode where your module can be turned into a self-contained RF system... eg:

• send a message when an IO line goes high
  
• change state of IO lines in response to received messages
  
  

Did you see the discussion about encryption here?

The TEA cipher looks great, and should be trivial to add to your radio system if you wanted it...

strings in Arduino are hellish to work with compared to Espruino.

:)