deskthority

On second thought, the daisy chaining and microcontroller are optional.
Given four connectors rather than two, the single-key PCBs can be arranged in the traditional scanned matrix.
The only active component is then a diode.

Also, you can skip the mounting holes on the PCB, and use panel-mount ...

The solution to the PCB size problem is this:

One PCB per key.

This PCB should hold only:

one keyswitch.
the cheapest available low-pin-count microcontroller (for instance, the PIC10F200, which comes in SOT-23 packaging!)
two small connectors for daisy chaining (simple one or two-wire bus)
two ...