Generally building UKIs is the intended output, but the only thing that tells systemd to pivot into an actual root is if it boots into the `initrd.target` unit,

Generally building UKIs is the intended output, but the only thing that tells systemd to pivot into an actual root is if it boots into the `initrd.target` unit, which by default it does when `/etc/initrd-release` exists. Other than that USIs and UKIs aren't very different. To answer the question if its planned: yes. It is officially supported to build USIs, just its not an explicit output format since its just a slightly different UKI (from a systemd perspective).