Hi Ro-nin,
There is already this capability in TG2, although it needs some work before it is fully realised I think. Check out this screenshot :
[attach=#1]
In this I've added a surface layer, made it green and am adjusting its maximum slope. The effect of this is displayed in the little preview in the top left, next to the shaders list. You may have to zoom out a bit before you can see the terrain. I have also opened the Surface Layer 01 preview in a new window, something which is in the current alpha. That lets you see things a lot more clearly.
In many ways this is better than the distribution preview in v0.9. For one thing, it's in colour ( although the greyscale preview in v0.9 was sufficient for distribution ). You can resize it, which is a big plus. You also rotate it in 3D, so you can change from a top down view if you want.
It's a little bit slower to display, but then TG v0.9 only had to preview on a heightfield, whereas TG2 has much more complex terrains. This should be something which can be multithreaded in the future, so it would be a lot quicker on multiprocessor/core machines. It would also be handy if you could pan around the preview area, at the moment it just zooms in and out on the origin AFAICT.
Regards,
Jo