Progress!  I've established that it is an analog signal, and it is indeed 9-pin VGA of some type (or something very close.)  The signal is just not quite right, but I don't know how.  As you can see, it's a bunch of zigzags interleaved, but you can definitely see the cursor!  I tried it on a couple of different LCDs, and I got the same screen on it.

The software (Dual Screen Software 2.E, found in the Lapis bundle posted here) is expecting an Ehman Full Page Display.  Don't know if the signal is compatible with the Radius and/or Apple Full Page displays.