Texture scale on terrain

I'm attempting to duplicate a technique used by the source engine to give a 3D Skybox.

In short, I have my normal (2,000 x 2,000) terrain which is used for the game as usual. I have a second terrain (100 x 100) a long way off (outside the normal render distance) which has a camera in the center.

When the main camera moves, a camera in the mini terrain is moved by 1/20th of the same distance. Rotation is applied unscaled.

By overlaying the cameras, I get a Skybox which allows for distant mountains, etc... which move appropriately with perspective.

This works very well and gives a nice effect with one caveat. The texture scale on the mini terrain is the same as on the real one - meaning that the distant mountains appear to have a texture size 20x larger than the main terrain.

Since scaling a terrain in the usual manner seems to do nothing, how can I scale the texture that is applied to it?