I'm back to report that I have the overworld map for this game mostly finished. The problem I've come across is figuring out how to walk through walls (or in this particular instance, walk on water) since there are a few areas that are very far away from the main islands of Japan that are difficult to place accurately if you play through the game normally. I should try again with the hacking to see if I can come up with something, but so far I've had no luck.
In the meantime, I suppose I could release what I have so far of the map, as rough as it is.