This is partly a question and partly a suggestion.
1) Question: Extracting and using the RW built-in textures to texture custom objects for RW would be considered a copyright infringement? The use case is to ensure a good blending of custom objects usable as new block shapes with built-in blocks (as an example, a column section or a capital used together with regular blocks).
2) Suggestion: It would probably be useful to have a way to access the built-in textures directly (i.e. without extracting and duplicating them); this would reduce memory usage for textures (whichever type of memory) and ensure a good match with surrounding blocks (UV mapping would be a little tricky, but I believe it would be worth the effort).
3) Suggestion bis: in the future it would be great to have a way to parametrise the texture used by a custom object, so that the same mesh could be textured at will with any of the textures available in-game at build time rather than at modelling time.