pkg_add -r libxml2 causes the webgui to fail with a 500 - internal server error. I used the FreeNAS-i386-LiveCD-0.7.1.4967 and did a full OS install prior to executing the pkg_add command.
libxml2 is already onboard (required by PHP and xmlstarlet). I assume that the port will damage something during installation. Badly to say but FreeNAS is not designed to be customized or enhanced via ports.