Turns out, if you trade over a pkmn from HGSS to DPPt then back to HGSS, the met location will go from "far away place" to its original johto/kanto met location! It seems this information is nowhere on the internet because I don't think anyone cares about this type of stuff, but that's still awesome to know! I just did the experience today and I couldn't believe it was real, I was expecting that the met location will be stuck as "a far away place" forever but it didn't!!

The shiny kingdra that I found in a old hacked soulsilver savefile was transfered back to his orignal game, and the met location was revealed: Seafoam Island, which means there's still a possibility that it might be legit! :D
That's as far as I can go for it unfortunately, I can't send it to pkmn home due to the DS online service shutting down, so I'll probably never know if this guy is the real deal but that's fine.