Use the "bake texture layout" plugin:
http://www.independentdeveloper.com/...texture-layout
- Make your sculpted prim
- In AC3D, duplicate your object
- Remap your duplicate with the texture coordinates you wish you had, can be anything you like
- Texture the duplicate
- Bake the texture from the duplicate onto the sculpted prim
- Upload the result of the baking and apply it to your object in SL