I've just merged the older LCD panel code base with my newer keyboard/LED code, and also connected the LCD display to the new controller card for the first time. To my big surprise it worked immediately! Finally both the LCD screen and the keyboard are up and running at the same time. And it only took me about 3 years ;-)
I also constructed a new cable and added connectors to the LCD end as well, as the soldered-on cable started to show some wear and tear. As the connector on the controller end is a normal ribbon cable connector, but the other end requires a 2,54mm spacing of the cables, I had to add to come up with a special solution. I am very satisfied with the end result, both in terms of how the cable looks and how it will fit the system.