It turns out that the offset was actually just a location in the ROM, I just didn't catch it before as I was comparing White 649 with the regular White and most of it was unchanged.
To actually do the wild data yourself, you have two options:
Go to the offset 083CBB44 in the full English ROM, OR extract the a/1/2/6 narc, which contains the same data.
If you use 083CBB44, you will be taken straight to the Route 1 data. The wild data follows on from there although it doesn't appear to be in any order apart from loosely by routes.
The data of each possible wild encounter is stored in the format of XX XX YY ZZ, where XX XX is the Pokemon's index number in hex flipped around (ie Patrat, whose dex number is 504, is 01F8, which is flipped around and entered as F8 01 in the ROM). YY is the minimum level that the Pokemon can appear at (in hex, of course) and ZZ is the maximum level. There will be a relatively equal chance of it appearing between any of those levels inclusive.
The amount of slots is the same since the ADV games. Wild grass encounters have twelve lots of XX XX XX XX, with each group of four bytes referring to one slot. They are, and their encounter rates as follows:
Slot 1 - 20%
Slot 2 - 20%
Slot 3 - 10%
Slot 4 - 10%
Slot 5 - 10%
Slot 6 - 10%
Slot 7 - 5%
Slot 8 - 5%
Slot 9 - 4%
Slot 10 - 4%
Slot 11 - 1%
Slot 12 - 1%
Following the wild encounters there will be another 48 bytes which refer to the encounters in the doubles grass (they will be displayed as 00 00 00 00 ... 00 if no tall grass is there), then another 48 bytes referring to shaking grass encounters, both of which have the same encounter slots as above.
The next 20 bytes refer to Surf encounters. Their encounter rates are as follows
Slot 1 - 60%
Slot 2 - 30%
Slot 3 - 5%
Slot 4 - 4%
Slot 5 - 1%
After those twenty you will have another twenty referring to the rare spots, with the same slots as above. Following that, there are another twenty bytes for the Super Rod, with encounter slots the same as the Surf ones I believe. The next twenty after that refer to rare spots when fishing.
If you happen to go to any sections that also have dust clouds or dark spots (ie Driftveil Drawbridge or Chargestone Cave) there are twelve slots for those, with the same encounter rates as grass for each slot.
If you'd rather just search for the beginning hex string of the wild data, it is "F8 01 02 02 FA 01 02 02" iirc
If you want to locate a particular place, just search for the hex string that will be in there. For example, to find Route 4, look for a Level 15 Sandile (this is easy to look at with Serebii's Pokéarth) which has a hex string of 27 02 0F 0F.
I have some offsets here that I've already located as I've been making notes as I progress through the hack I'm doing:
Main ROM
---
Route 1 - 083CBB44
Route 2 - 083CBC2C / Trainers - 1, 2, 85
Dreamyard - 083C652C / Trainers - 7, 8, (277, 293?), 308
Striaton City / Trainers - 9, 10
Route 3 - 083CBD14 / Trainers - 14, 15, 16, 17, 18, 19 (late), 20, 3, 4, 519 (late), 292, 84
Wellspring Cave - 083CBDFC - 294, 62, 63
Wellspring Cave B1F - 083CBEE4
Nacrene City / Trainers - 86, 87, 88
Pinwheel Forest Outside - 083C66FC / Trainers - 111, 160, 501, 502, 546, 144, 145
Pinwheel Forest Inside - 083C67E4 / Trainers - 295, 296, 297, 309, 26, 27, 28, 29, 30, 31, 32, 33, 34
Chargestone Cave 1F - 083C879C
Chargestone Cave B1F - 083C8884
Chargestone Cave B2F - 083C896C
narc
---
Castelia City / Trainers - 93, 94, 113, 114, 116, 165, 166, 167, 170, 137, 552 / 196, 198, 199, 200 / 600, 601, 602
Route 4 - 0000645C (.narc) - 36, 41, 42, 291, 505 / 35, 37, 39, 40, 153, 156, 290
Desert Resort Entrance - 00000CCC (.narc)
Desert Resort Main - 00000DB4 (.narc) - 233, 234, 235, 236, 237, 238, 239, 549, 550, 613
Relic Castle 1f - 00000E9C / 51, 52
Nimbasa City / Trainers - 279 / 66, 67, 68, 69
Route 5 - 44, 45, 46, 47, 48, 49, 50, 401, 402 / 000064B4
Route 16 - 408, 409, 412, 438, 442, 443 / 00008554
Lostlorn Forest - 0000863C
Driftveil City - 603, 168 201 202 203 95
Cold Storage / Trainers - 72-77 inclusive, 81, 140, 141, 610, 276, 278
I ended up swapping to the narc about halfway through so the offsets are split The numbers next to each place are the numbers of the trainers of that area in KazoWAR's Trainer Editor. I've slashed a couple off; it's usually to do with when you fight them ie the ones after the slash in Route 4 can only be fought after defeating Burgh.
Hope that helps