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Just as a follow up, I tried the method with one bright green LED as the 'on' state and two very dim orange leds to indicate 'available'. When on, I can still see a bit of the orange on the edges, but there's a distinct difference between on and off, and that's most important. I will upload some real pics of this when I have a moment.
In terms of wiring, the green led will be wired to an LED64 board. The always on led will simply be hard wired to the power supply.
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Another option I have not seen listed here:
Use a tri-color LED and switch the polarity for ON, OFF or FAIL.
A few more parts, but if you put all the switching circuits up front of the matrix, it won't be that much more complicated.
You could have:
RED = Alert
Blue = Normal
Green = TEST
OFF = Off