Chip package type is WLCSP with diagonal pin pitch 0.4mm. Luckily boards don't appear to have more than 2 layers, and size of vias suggests that all pins are fanned out to the sides.

After careful examination under microscope I've identified following pinout of ribbon cable:

1. GND
2. ?
3. "Hidden" button
4. ?
5. SCK
6. SDO
7. CSB
8. SDI
9. ?
10. VDD_IO 2V - voltage is dropped to 1.6V for VDD with local regulator
11. ?
12. ?

There are no points on the little pcb available where wires could be soldered without removing connector. Luckily all connector pins have a test points on the next board that ribbon cable connects to.

To confirm pin assignment I tested them with oscilloscope. SCK and CSB pins checked out, but there is no activity on SDI, and SDO.