Raspberry Pi has an audio jack, so you can hook up speakers and write a simple DroneKit script that will parse the system’s status and play a tune of you choice when somethin’s up.
You can also connect a buzzer over I2C and build ArduPilot with the needed support. Just add ADD_BACKEND(new Buzzer()) in here.
I very much like the DroneKit idea. In fact, it will let me do few others things that I want to do in terms of diagnostic. Very cool!
Just to clarify, you are talking about built in LED on NAVIO board, right? Problem with that is depending on how the Rpi + Navio is caged and placed in my rover, it is not very suitable to be exposed / easily visible from outside.