Christmas slowed down the progress of this project a bit, but today I connected up the latch-"board" to the display&keypad module.
It was bit hard to solder up, but I think it ended up in a not too bad shape - there are quite a lot of connections between the two units so some creative routing was necessary.
I also had to forego my initial plan to have the displays and segments in an easy ascending bit order, they ended up a bit scrambled but that is not a big issue since it's easy to fix that in the firmware that refreshes the display.
At the backside of this module I added a 20 pin female 0.1" header to connect to the CPU module with some flexible wires. Having them soldered permanently attached with AWG 18 wires would not be fun during building and testing.
I hooked the databus and three control signals up to an Arduino and wrote a short program to test the displays and the keypad. It worked just fine after I added pull-ups between the keypad and the 74245 transceiver. I had overlooked that floating inputs of the logic family of the 245 I was using did not consistently read high as the old LS-series would do.