I don't think just adding the texture files to the .s3d archive is enough, you need to reference them in the .wld file as well as you mention. I think you can do this by creating extra 0x03 fragments that reference the texture file names and 0x04 fragments that reference the 0x03 fragments. If you haven't already you should have a look at this
document written by Windcatcher.
This is just a guess as I don't know how the official client deals with this. As far as I know there is nothing in the .wld files that says how many texture sets there are for a given character model.
If you want to try out your changes quickly and not have to fire up the game every time you could try out the
character viewer I have been working on. This should give you an idea of how a drake would look with your new textures in game, though of course it doesn't work exactly the same than the official client.
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