Creating a mask based on distance from world y-axis

[Badbullet]

In 3ds Max, I'm trying to make a mask that goes from white to black depending on how close a surface is to the world's y-axis. Falloff by distance blend set to world axis doesn't seem to do anything. I swear I used to be able to set it to a world axis in the past to make these sort of masks. Distance to an object won't work, because I need it consistent across the xz plane. Any thoughts? Is there possibly an OSL solution?

[Badbullet]

Looks like VRay Distance Texture renders fast enough. Was hoping to avoid it, but until I find a better solution it'll do.