Expose FFS as the native OpenBSD filesystem option for volume creation and accept FFS/UFS on the command line, mapping mounts to the OpenBSD ffs filesystem type.
Run newfs through the elevated core service on vnd raw devices, then temporarily mount the new filesystem to transfer root directory ownership back to the invoking user.
Keep the non-interactive creation default as FAT on OpenBSD so existing unattended scripts do not start requiring elevation.
Mounir IDRASSI