Have a landscape
Have a foliage object. Set it up so that it paints only into a certain landscape layer and se tthe foliage paint tool so that it paints only on landscape
Place a blocking volume on the landscape outside of the area that is painted with the landscape layer
Paint foliage.
The foliage will be painted onto the blocking volume even though this is not on top of the desired layer.
Even if it were, painting foliage on a blocking volume (or other volumes) is undesirable.