That's what I thought all along, fastboot is like the swiss knife for running custom code
In android it can also boot custom images like "fastboot boot boot.img", can the one on kindle do this? If it's the case, easiest way to make a recovery tool would be to create a custom recovery image with tools to export partitions over usb, and even performs automatical checks. This could also allow installing additional code using flashable .zip, like on android.