zfsboot patch for /usr

Wondering if anyone has example patches for zfsboot (from
usr.sbin/bsdinstall/scripts)?

We're looking to change some of the default zfs subvolumes, removing /usr in
favor of /usr/local in particular, and have run into a "parent does not exist"
issue.  It's not clear where in the script the /usr parent dir should be
mkdir'd.

Roger