The plan for now is to create a development board with the ESP12F module on it, and one of the USB-enabled microcontrollers. The board would have a special "files" button on it. If started without the button pressed, the microcontroller will act as a regular USB to serial adapter, pretty much like the ATmega8u2 o the UNO, letting you program and debug the ESP8266. However, if during the startup the button is pressed, the microcontroller will keep the ESP8266 in reset, connect to its SPI flash through the pins on the bottom of the module, and make it available as an USB disk. This is the minimal plan.

Now, how to get there? I'm thinking about the following steps:

• get the blinky demo to run on the microcontroller,
• get the USB serial echo demo to run,
• get the USB to serial working,
• add AT commands for changing speed etc.
• get the reset and programming logic working,
• program the ESP8266 with it,
• get SPI working,
• communicate with the on-board flash chip,
• come up with a way to find the beginning of the filesystem on the flash (may require manual configuration),
• use a FAT library to verify we can read the filesystem correctly,
• get the MSC demo to work,
• modify the demo to use the flash chip,
• make it use the correct offset,
• run both the serial and the MSC depending on the button pressed.