as a houserule, back in 3.5, we allowed items with the same function for different slots as long as it made a general 'sense' (i.e. boots of Dex). As for your question, i would say that it's ok to allow an amulet of sustenance, as it is still jewelry and has the right feel to it. goggles of sustenance, on the other hand, have not.
i think there are currently no rules on certain functions on bodyslots (correct me if i'm wrong), so broken down to a rules question, both examples should be valid.