Would anyone be interested in developing a USB peripheral backpack with me?
The main aim is to make the Raspberry Pi a USB peripheral of a (more powerful) USB host, such as a PC. The backpack can power the Raspberry Pi via P1-02 and communicate over the SPI and/or UART interfaces (jumper configurable?).
I was thinking of using an ATmega16U2, but I think I'm a bit out of my depth designing a decent and robust circuit, routing a PCB and testing it. For example, AVR chips have a standardised 6-pin ISP header based on SPI. So, the Raspberry Pi should be protected from the ISP header when it is programming the chip. Furthermore, if designed correctly, the Raspberry Pi could program the backpack itself.
This idea was inspired by How can the Raspberry Pi emulate a USB storage device?.