I took an area of Episode One and gave it an ash pass using only several hand-placed ash stain overlays on the brushwork, and a shader that dynamically places an ash layer (blending a diffuse texture and a separate bumpmap) on world geo and models, with tweakable settings for ash direction, amount, transparency and tinting.
Because not a single texture had been repainted, it only took 1.5 megabytes (for a shared masked ash texture and edited stock VMTs) to achieve the result.
Not gonna lie, the ash pass thing makes some details there look slightly better which could make the maps slightly look if it wasn't cleaned so much.