Yeah, we’ve considered adding support for publishing to split images for a while but it’s never really made it to the top of the list of features to implement 
Scripting the repacking process should be reasonably straightforward though:
- tar xvf unified.tar.gz
- mksquashfs rootfs/ rootfs.squashfs -noappend -comp xz -b 1M
- tar Jcf metadata.tar.xz metadata.yaml templates/
You’ll then get your metadata.tar.xz and rootfs.squashfs for a normal split image.