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

Sounds like a really good idea...

I'd suggest a (maybe optional) RTS/CTS implementation (it could always be removed by shorting the 2 wires together). It'd allow someone to use the setDeepSleep mode of Espruino, with a small bit of wrapper code:

setWatch(function(e) {
  digitalWrite(CTS, e.state);
  setDeepSleep(!e.state);
}, RTS, {repeat:true, edge:"both});

Just some ideas:

• Maybe go for AT commands? That seems pretty standard for serial
  
• Ciseco had their LLAP protocol. It's a similar idea, and might be worth a quick read just to see if they had any ideas
  
• As in LLAP, it might be worth making sure $(data) is actually just hex characters - it'll make it a lot safer (for instance how would you send \r otherwise).
  
  

Finally, I know this goes against your plan a bit, but I whether anything could be done in Espruino to make it better at decoding RF... Maybe I could add the ability to drop any state changes that happened too quickly, before they even got to JS.