I use WM for generating masks, and while TG2 has a lot of improvements, WM is still very useful.
http://forums.planetside.co.uk/index.php?topic=864.0This post looks at getting the correct result from image maps used as masks and may be of some help to you.
If you look at the node network from my main results section you might also get some more clues in combining masks.
As for using just shaders, here is abasic example of what I do.
1: Create surface layers that will later act as the mask using just white and black (or greys if you want to get tricky)
2: Create a surface layer with the desired colour, removing its default fractal breakup
3: Bypass the mask "surface layer" and connect it to the fractal breakup of your surface, adjsuting these settings in your surface: Coverage = 0.5, Fractal breakup = 1
4: (optional) Insert a colour adjust shader to provide finetuning of the mask.
The attached TGDs demo each of these steps.
Having done this for each of your surfaces the other shaders that will be of use are the colour functions. These can be used to make combinations of masks from other surfaces for a different surface layer. In my image mask fix post linked above, for example, I use a WM flow map image from which I subtracted steep slopes and high altitudes (distribution shader), from which I subtracted my water distribution mask (image of lake surfaces) to give me a density shader for my tree populations.
Hope this helps.
Ben