To answer one of my own questions:

The LX7000 second-seat DIGITAL slave unit cable pinout:
It's a DB15 wired to a DB9 RS485. The (undocumented?) DB15 pinout is
similar to that of the ANALOG main unit (and also somewhat similar to
the main digital unit):
1,2,9 = ground shield
7,8,15 = +12V (red and white)
5,12 = RS485 data bus (black & yellow)
11,3 = serial communications - mine came with these pins wired to a
5-pin round "PC port" connector, not clear what functionality does this
port offer.

Curious whether any other of the 15 pins are active in this unit, e.g.,
SC and VP switches, speaker output?