I have a Raspberry Pi, Teensy 3.0 and 3.1, at least two working servos, a shift register and all the stuff I need - apart from the Pi-Duino-Teensy board, which is somewhere between the US and my mail box. Meanwhile, I can write code to use the Teensy 3.1 as an I²C slave, and to generate the PPM signal. Both features are basically provided by teensyduino libraries, so this should be fairly easy.