Landscape painting; how do I create a layer with an alpha mask?

So I’ve figure out how to set up different material components as individual brushes for painting landscapes, but the layer blend node in the main material doesn’t appear to have an option for transparency, how do I go about it.

So I have a texture with individual rocks on it, with a transparent background. I can set up a normal plane mesh so you can only see the rocks, can set up a landscape material so one of the layers only paints the rocks and not the black background in a tiled fashion?

Thanks,
James