Don't be so quick to judge, I've got a small FAT partition on my boot drive right now for the EFI system partition. That partition also doesn't frequently have data written to it, you could in theory hide a small encrypted volume in the free space and so long as Grub wasn't updated nothing would touch that free space.
I don't think FAT even bothers to zero the data when it's allocated, so you might be able to see what was previously stored in a block by reading it straight after allocation.
