timb, which FRAM module are you considering. Cypress also has the 256KB FM25H20. I agree with you on choosing FRAM over others. I am not sure why you would dedicate some SRAM though. We have the 1.5MB of external flash (once the library is finished) and a 256KB FRAM/microSD combo would seem to fit the slow (microSD) and fast (FRAM) requirements.
What would be cool is to add an MPS430 that makes the FRAM act as a fast cache to the microSD. Or act as a microSD to FRAM loader to save on lots of I/O. The Spark could send commands via I2C to have the MPS430 fetch a block of data or file or whatever from the microSD and load the FRAM with it. You would have to isolate the SPI bus from Spark’s during the transfer so as not to hog the bus. Just thinking out loud mind you. 