I wrote some code to do it (it was rather simple) but something was preventing rockbox from booting, I didn't try too hard to debug it since at the time there were changes made to the I/O (because of raaa mainly) and the simulator.

But I think this is a nice idea, not only for FAT but for the storage layer in general.

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 This actually sounds similar to the emulator project.  Perhaps some combination of emulating the CPU or simulating the storage layer would make a good project.