I'm not convinced that you're saving anything by avoiding nbd. You need to modprobe nbd and start qemu-nbd. Both single commands that you can cut and paste. What you gain is not having to convert an image at the end.
I am with mwcampbell. There's something to say for the minimalism/lack of dependencies - even if it is not much more complex to use nbd. You don't need qemu to create a sparse file, mount it as a loopback device and manipulate it as a new device from there. If someone needs a qcow2 file afterward, fine. But that doesn't /need/ to be involved in the build step.
