Huzzah! I have the answer! Or something. =p
Okay, I used the LUA scripts here and a bit of hex searching to isolate the overworld co-ordinates in hex. If you use the script (much thanks to Kaphotics for that) to get the X and Y co-ords of an overworld, then search it in hex in reverse (i.e. co-ordinates of X: 40, Y: 30, you would search for 28 00 1E 00) you should be able to find it. The relevant .narc is a/1/2/5, which will make your search a lot easier.
Here's some evidence; I edited which overworld sprites were in the Darmanitan area (BA 00 corresponds to the statue sprite, apparently):
This is something I've been wondering about myself, so hopefully that helps!
For the record, a sprite ID that isn't known will result in a generic boy with blue hair. And from random experimenting (not including the ones in the pic as I just punched in odd numbers so I don't actually know which correlates to which ) I found Celebi to be ID number 4380 (so 11 1C in the hex) and the other hgss sprites follow before/after like usual.
Never mind that; couple of hex values for sprites
D0 - Gift Box (from when you receive starters at the beginning)
D1 - Cottonee
D2 - Poke Ball again? Closer to the ground, though...
D3 & D4 - Reshiram
D5 & D6 - Zekrom
D7 - Cynthia
D8 - Poke Ball (in the air..?)
D9 - Deerling
1C11 - Celebi
I assume there's an easier way to find this out than trial and error, but some starting values there I suppose. And of course, just change the co-ordinate values if you're looking to do that.
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And a look at the hex string of any one overworld, using one of the Darmanitan statues as an example...
06 00 BA 00 00 00 00 00 98 02 07 00 01 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 3B 00 23 00
00 80 01 00
This is presumably the ID of the overworld.
This is the sprite ID.
I believe this corresponds to the movement?
I THINK this is the flag...
This is the script ID, corresponding to that map's scripts.
Direction the overworld faces. 0 = up, 1 = down, 2 = left, 3 = right
I have a hunch these decide how far an NPC goes if their movement byte allows them actual movement
X co-ordinate
Y co-ordinate
Z co-ordinate, I think?
As far as things go, the layout seems identical to all five of the fourth gen games, although given it was based on the same engine it's not that surprising.
I better go back to exam revision...
Oh and one last thing, the LUA script also displays the map number, and the files in a/1/2/5 are divided between each of the maps so you should be able to relocate things pretty easy. The Desert Resort is #158, for example, so all the Darmanitan data (and others) will be in file 158 of a/1/2/5 and so on. I tried a little script copypaste and it totally failed, though, so I guess overworlds is all we can change for now.