It would be perfect If I could tell the VRay User Color node to read regular Maya Attributes.
This is already done; in recent nightly builds, VRayUserColor and VRayUserScalar textures will recognize Maya attributes if they start with “vrayUserColor_” or “vrayUserScalar_” or “vrayUserString_”. For example, if you add a dynamic attribute with the name vrayUserScalar_test, you can then use “test” inside a VRayUserScalar texture to access it.