Ok, figured it out. It was the 4049 quad nor gates.
I tried my logic probe up-stream at the PIA and other components and got a pulse signal at the output pins as expected. I moved on to display controller board. I figured it was not the hex buffers which control the segments since I observed most of the segments working on both the top and bottom plasma displays during the display diagnostic test. When I probed the nor gates that control the strobe signals for each digit however, I still seem to be getting something at the outputs. Strange.
I decided to try and remove, socket, and replace the nor gates in different positions, to see if the always working digit moved from the right-most digit to another position. Bingo. I used my hot-air solder gun and it took only a few minutes per chip to remove, solder suck out the holes, clean up with solder wick, and place sockets in all the logic chips (4049s and 4001s). I ordered replacements from Jameco for less than a dollar per chip and display worked again! Mostly...
I had a few segments out on the bottom display and upon inspection discovered that my roughhousing of the display leads to get to the chips had broken off one lead flush, and caused two others to detach from the glass. Bummer.
For the lead that broke flush, I applied some solder paste to blob some solder on, and gently attached a fine wire. That fixed one segment.
solder blob on flush broken lead
The other two were harder because the actually detached from in-between the glass sandwich. I made a wire lollipop and applied more paste to the terminal and the lollipop hoping I could wedge the lollipop up against the terminals. It worked, and I have all my segments back!
Fixed detached leads with lollipops made with a tight wire spiral at the terminal end
Since I'm convinced the whole problem happened because I shorted out the logic elements while doing another repair, I placed a paperboard insulator on the back of the display. Overkill for sure, but I don't want to ever do that again!
Thanks for the help and suggestions!