Thanks for the suggestions. The BIOS POST codes led me to:http://www.computerhope.com/beep.htm#06
which has interesting alternatives. Some of these are Morse-like in that they consist of 3 or 4 grouped shorts and longs.
I'm not overly impressed with the idea of say nine, ten, eleven shorts in a row, much room for error. I've no doubt Marconi considered a scheme with 1 spark for A and 26 sparks for Z. Also found other pages for different devices each with their own scheme, many based on Morse like sequences.
My application for this is a standalone UNO+SD - if in setup I can do my diagnostics and at the end of that ping out 3 short flashes for success then that will be enough to reassure me that it's doing its thing. After that I am confident it will keep on track. I have already found that pin 13 is bad for this signalling, as soon as the SD is initialized it seems that pin 13 is taken over since flashes become garbled. Pin 8 seems to be ok with its own LED+resistor.
I envision at least a few useful common codes:
three shorts for end of setup and successful launch into loop(),
continuous wide spaced single shorts (Morse E) for fatal error (the BIOS POST continuous tone is not a good idea unless it is warning of nuclear attack),
a long and two shorts (D) might indicate a disk error, plug in card, etc.
Yet another application-specific scheme I guess.