I'm not following this but something seems strange.
Are you really saying that a program that is in /sbin will not work with
/usr not mounted? That seems quite broken; the whole point of /sbin
being separate from /usr/sbin was always that things would work without
/usr.
So it seems more reasonable to conclude that dhclient must be excluded
from the sanitizer than that it should fail without /usr.
(There's the whole issue of whether ISC dhclient is so big that it
doesn't belong in sbin, switching to dhcpcd, etc., but those are
separate issues.)