Finally made it. With help of Eagle autorouter plus some re-wiring. I have a lot of clearence alarms :(
I understand that it should be polished, but at least I can say here it is, it is possible to do this thing.
I know it's going to work because I've researched the following topics
- Using Shift registers for keyboard-like device in raspberry Pi, with help of WiringPi library
- Using SPI OLED monitors with Raspberry and doing 180 degree screen turn in them
- Programmiong Arduino from Raspberry via serial port (tricky in terms of software config, but should work)
- As a backup plan - I can create an expansion board to programm Arduino via ISP port
- As another backup plan - use USB to FTDI convertor (although I'm afraid this board is done, hardly I can squeeze anything onto it)
I still have a lot of questions to answer. I will try to test the concept of using on-board keyaboard while connecting via Mobile phone remotely. And I need to decide what to do with power for Raspberry.
Here are some rederings of the board