frank2644:
Your SP1/I2c Uart suggestion is a good one, and that inspired my idea to use another esp8266 for that purpose. Assuming the ESP can act as an SP1 or I2c slave as I think it can.
the I2C slave doesn't work on esp8266. the Wire library has it, but it is considered experimental and not documented. there are many reports that it doesn't work.
SPI slave is OK