From the website:

  FatFs controls the storage devices via a simple media access interface shown below.
  disk_status - Get device status
  disk_initialize - Initialize device
  disk_read - Read data
  disk_write - Write data
  disk_ioctl - Control device dependent functions
  get_fattime - Get current time

For your use case, you'd probably have to implement the disk_read method to dynamically generate and return your content based on the start and end sectors provided as parameters.