Further info, plus resolution, for future readers and posterity:
The CAN bus wire came loose during a program upload, and after this, the red LED was flashing at 2.5 Hz, with a blank screen on the DP600 displays.
If this happens, the screens will only talk on the same CAN pins as the ones they were using when the upload was in progress, and will appear in the service tool as something like "bootloader_mode".
So, if they were interrupted during an upload using pins 3, 4, and 5, you need to hook them up on your test bench with pins 3, 4, and 5; If they were using 5, 6, 7, use pins 5, 6, 7.
Once the service tool detects them in bootloader mode, just upload a DP600 program to them, ignoring/overriding appropriate warnings that come up about memory, etc.
After this, they will work normally.