We have added a new topic to the Krakatoa 1.5.1 Online Documentation discussing the use of Transparent and Semi-Transparent / Opacity Mapped Matte Objects and the ability to save foreground and background image layers for compositing.
While that does explain the concept, wouldn’t a better approach be to render the lighting to the emission channel then cull the particles with the mesh to two sets, those in front of the object and those behind, render each, render the mesh (with scanline or another plugin renderer) and composite the 3 together?
The advantage is that you can render as many layers as you need.