please, try with this lua scripted graph:viewtopic.php?f=73&t=63807
since it is not yet supported, it will not be saved correctly, but you can use it to obtain the render, I guess.
While in octane, select the desired Render Target/Scene view, then right click in the node inspector on any icon, and choose “Show in Graph editor".
Then right click in the Graphbeditor and create an Other/Scripted graph node.
Asdign the lua script, then copy paste the camera node from the RT to the lua camera slot, then load the background image, connect the Visible Environment pin and Film settings to the RT node from the lua scripted node, and set the desired settings.