I took a look in pkg_install/create/perform.c.  It passes -T - to
tar and then writes a list of files to its stdin.  However, it
apparently expects tar to interpret -C sequences in this list:

		} else if (p->type == PLIST_CWD || p->type == PLIST_SRC) {
			
			/* XXX let PLIST_SRC override PLIST_CWD */
			if (p->type == PLIST_CWD && p->next != NULL &&
			    p->next->type == PLIST_SRC) {
				continue;
			}

			fprintf(totar, "-C\n%s\n", p->name);

Oy.

--  
Eric Gillespie <*> epg@pretzelnet.org

Build a fire for a man, and he'll be warm for a day.  Set a man on
fire, and he'll be warm for the rest of his life. -Terry Pratchett