There is device meta data to set up as well. So you still need to create the MD device in the database, which creates the device number that the diskless image is under. So you can't copy the files until then anyway. And once you create the device by booting the MD it is going to go into the diskless image build process anyway. Also, there are likely unique identifiers in the images. And finally, you would still need to manually create the PXE boot initramfs image - that on its own is more effort than just allowing the MD image to build on its own. It only takes a few minutes anyway!