We already have the "data tag" support, which is used to annotate metadata. In eMMC 4.5 any request can be tagged either metadata or belonging to one of 5 to 15 contexts, or being unspecified. Any request that a file system flags as a REQ_META write uses the data tag.
Since FAT is both very simple and very common, a lot of the flash devices actually have logic in them to detect the access patterns even without those annotations. Usually the FAT area is known to be at the beginning of the device (partition-less SD cards and older USB sticks) or an erase block that receives lots of random I/O is expected to be the FAT. Further, devices often expect a FAT cluster size of 32KB, so all requests of that size are taken to be data while all smaller ones are treated as directory updates.