Woohoo! The keyboard is now assembled in the box, and most of the back panel connectors/switches are in place (it's just the foot switch connections that are currently missing). This means I now have a portable, customised-for-me gaming keypad *cheers*.
For the circuitboard, there were two "errors" that needed minor adjustment. The first was that the default footprint for a CR2032 battery holder in kicad expected a different distance between the battery terminals (longer than the holders that we had), so we needed to extend the positive terminal leg by about 2-3mm. The other issue was to do with how IDC cables/connectors work - I'd very carefully made sure that the screen would "plug into" the board, but without taking into account that I was planning to bring the cable out of the top of the board and thus I should have been plugging in the screen from the bottom of the board to get the row orientation right - plugging from the top resulted in the two rows being swapped. We changed to using a right-angle header underneath the board which then had the rows in the right pattern to match through an IDC cable to the screen - and I learned more about how IDC cables work in the process, so I know for future reference how to check those. In the end, that right-angle header worked better to fit the box anyway, so it was a win regardless :-).
The only thing still missing is that we haven't added the footswitch connector to the back panel yet. Still working out whether we'll do a panel mount connector, or whether we'll go with a flyout lead for that. I use the footswitches for push-to-talk in game voice chat, but if I'm at the bed PC I can't really activate a footswitch properly (maybe they become elbow or knee switches then...*laughs*) so for initial testing it didn't really matter. All the keys are remapped for their new wiring, and working.