Well, that worked. If anyone's going to combine these two methods, just one more thing to note.... don't use the same image mask for displacing the terrain to make the lakebeds and restricting the distribution of water. This will result in a narrow gap around the edge of the water.
Now I just have to break up my lake bed mask into individual lakes. Placing them is going to be a pain but the end result will be worth it.
Masked
[attach=#1]
Unmasked
[attach=#2]
Masked, but pesky trees on the water.
[attach=#3]
The distribution of trees is controlled via an image mask, and to be sure I also subtracted the water distribution mask from the tree mask.
I've noticed even without masking that objects will appear outside of the bounding rectangle.