You have made the assumption that the oxygen is supplemental oxygen, this may not be so. You have questioned the safety of having oxygen cylinders exposed, I suggest you ask the FAA (the licensing authority for NWA) rather than an open forum, that is if you really want to know a definitive answer.
It is only my opinion, but, I think that they do not replace the chemical generators for emergency oxygen. I think they are theraputic oxygen bottles or for crew use in decompression situations. The siting could be reconsidered, why not ask NWA if this is possible? Let us know what you achieve.