Regarding getting the buzzer moved to Y/O pair on Connector 1 of the call director, I found I could make the following simple wiring change that just involved moving a few wires around:

[Linked Image from seriss.com]

(If you spot any corrections needed, I can update that diagram)

I finally got the courage to dig in and search for the terminal board inside the phone, which was so buried under the line button strips' wiring harnesses, I didn't even see it at first. Seems the short procedure to access the board is:

1) Remove the plastic case
2) Loosen the 6 screws holding the 3 line button assemblies (a screw at the top and bottom of each)
3) Flip over the button strips with their thick wiring harnesses to access the terminal board

Quite a lot of wires in there, lol.. here's a pic of my Call Director's terminal board; 50+ pairs of screws in there, woo:

[Linked Image from seriss.com]

Was able to identify the two terminals the buzzer was wired to easily enough:

[Linked Image from seriss.com]

..and connector 1's O-Y wire that was spared off (using a multimeter to find the right one):

[Linked Image from seriss.com]

..and connector 1's Y-O wire which was wired to terminal 3.

Since my 1 board KSU uses 12V to run both buzzer and lamps, I was able to wire the Hold lamp across the buzzer as well (Y-G pair) so that it lights up when the extension is buzzed, which should be good for the 8 phone / 4 line demo I'm preparing.

So all appears good for the buzzer mod; the phone buzzes and rings just like my others now.