Thanks, mcdermd!

Over there, back then:

I dismantled my Apple Keyboard II to have a look, and I found that the keyboard matrix is joined onto the electronics board by two ribbon cables, the left has 13 connections, and the right has 14.
On the Duo keyboard, two ribbons, left 13, right 14

Click to expand...

Hm. Unless some of those are ground, power or something else (which seems likely) even the 20x12 Key Warriors won't have enough IO. More than that though, I'll need to find out which are which, or bypass the ribbon and head straight to the matrix PCB (possibly the better approach).