The HAL/Cube example trees have examples of Host MSC, typically using microSD. There might be some SDRAM based RAMDisk examples, there are for FatFs, and converting them to QSPI shouldn't be a huge leap. The DISCO/EVAL boards have QSPI BSP code.
For Read-only the memory mapped modes of QSPI should work, for write you're going to have to either switch modes, or stay in the the non-mapped mode. You're also going to need to erase blocks as needed.
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