@Evan J mentioned he had a problem with switches in the top piece, so I though I would have a look at my planned mod and see if it would fit. There is not a huge amount of room as shown below, but not as tight as the screen will be.

I'm using a 12mm tactile push-button and inserting it underneath the existing button. Originally I thought I would cut the existing button down to size and fit in beneath the spring, but woudl be tight and risk damaging the visible button piece if I screwed up so I went with mounting in under the existing button, and drill a hole for it to poke through. 

Exploded stack-up: - button - kit holder with hole, kit spring, kit knob, kit button closure

I executed this poorly by trying it at home with a hand drill,  instead of making a jig and doing it in a pillar drill at my local maker space, so mangled the piece a bit :( Still managed to salvage it and it looks like it works fine - maybe a millimeter to be removed from the button stop.

Close-up of the modified button holder with the button poking up through the hole 

I tacked it in place with super glue and then went around it with hot glue to secure it. In time I might go for an expoy for a more permanent fit. I might have to 3D print a replacement if the damage to this one proves to great...

Loads of room, as long as I don't try to put a hat in there ;). Using the GPIO pins down that end may require soldering wires direct to the pi instead of headers and jumper pins. It will certainly be tight once all the wiring converges on the pi - maybe a GPIO expanded on the I2C bus would simplify the wiring of all the buttons?

The button no gives a satisfying click each time it is pressed, and I know it will work just fine with the pi. The three STAT/ITEM/DATA buttons will get similar treatment but with smaller 6mm push-buttons, but I'm going to do that more carefully so will have to wait for another day.