Well from my search I came up with this:
This key opens the ornamented door in Sea Serpent Grotto. You find the ornamented door at J-11, the gold beastcoin door at I-14, and the silver beastcoin door at N-14. You use this key, then it breaks.
You need a silver beastcoin, gold beastcoin, 3 mythril beastcoins, and a norg shell (found in AH under other > misc.) when you enter SSG. You need to do the entire SSG section without leaving the zone, otherwise you have to start it over.
First you talk to a Gimb, a galka on your right as you enter Norg. Then you find Bou the Righteous at H-11 through the Gold beastcoin door. He mentions Hurr sells keys, so go to Hurr the Betrayer at H-3 through the Silver beastcoin door, the same room you go to for the Utsusemi: Ichi quest. Hurr asks for a Norg Shell and 3 Mythril Beastcoins.
Hope that helped.
EDIT:
In case you're wondering I found this info from a post on
Allakhazam
Other then this I don't see much else, no clue what's behind the ornamented door.