Concerning the BLE, I'd be super interested in implementing this as well! Actually, I am quite far already with a small numpad I use for testing purposes. Aside SPI, which is definitely needed for this, are there any other GPIO pins broken out? I think two or three would be needed to properly control the NRF module that Adafruit uses.
[Edit] If course, I am very interested in a PCB and maybe a plate, too
Pins broken out on headers:
MOSI, MISO, SCK, SDA, SCL
Pins easily accessible but not on headers per se:
RGB data (you'd have to solder to the Din pad on RGB1), num lock, caps lock, scroll lock
Due to routing issues, they weren't brought out to points but there are unused pins on the MCU you could attach to if you're skilled enough:
PE3-6, PB0, PB4, PB5, PD5, PD6