So just after presidents day we came back into the office and found that the FLED was dead! Turned it on and nothing happened. After eliminating the ATX PSU from the equation it became clear we would need to pull the display down and open it up to find out what was wrong.
Once we got it open we discovered that the PSU was tripping out, evidently from a short in the LED chain. We disconnected the LEDs and everything started up again. So it was time to find the short. Since we'd constructed this string of LEDs ourselves it seemed likely that a connection had broken or shorted. We carefully went over every connection and found that none of them were obviously broken. Therefore it must have been the LED units themselves.
Because we're using WS2811 driven LEDs it didn't seem like there was any easy way to debug the LEDs individually. So we had to resort to a binary search, this meant cutting the strip at various points to find where in the chain the issue was found. After 7 cuts we managed to isolate the one LED that was causing the problem. After tearing it off the backplane we were treated to this lovely example of what happens when you have lots of heat sealed inside some silicone!
Once we replaced the LED and reconnected the cuts we made FLED was restored to all its blinky glory. If anyone has any better ideas about how to debug WS2811 strings we would love to hear your thoughts.