The next plugin build is going to have this new feature:
Added a new "visible environment" pin to the render target node. If specified this environment is used for reflections, refractions and all camera rays that leave the scene and the regular environment is used only for the direct light calculation.
But I am not sure if this is what you want. Are you talking about some kind of compositing function using the alpha channel to add a background plate to the rendered image?
+1 Flat background plate is one of the nice things that I used often in octane for cinema 4d. But don't have in houdini's octane plugin. This function would be very useful and handy.
This feature will be available probably once we have ready the new Qt based IPR, but meanwhile, how about using MPlay as IPR viewer when you need to work with background plates? ... I know that the MPlay IPR has some limitations, but on the other hand, it supports the background plate feature without problems.