well...use that damn distance shader
Seriously: attach your render camera to the camera input of a distance shader.
Set the near and far distance to 1000m. (If you want a small region of fade-out of the grasses then set the far distance to 900m for example)
Now you have made a mask for anything within 1000m from your camera.
You can now use this distance shader to restrict populations only to within 1000m from your camera.
Since the camera is moving in animation, the distance shader moves along.
But keep in mind that you'll need to make a population area which spans the entire camera-path.
This will use much RAM, so perhaps you should try to move your population-area as well.
If you keep the same seed of the population then you should not be able to notice that the actual population area is also moving.
If you want to use slope/height restrictions as well then do the following:
Create a surfacelayer, pure white color. Set the slope and height restrictions.
Use the distanceshader as blendshader.
Use this whole as density shader for your population.
Hope this helps.
Cheers,
Martin