Only a couple of hours after my post above I finally found the proper solution for a flat backplate image in Blender Octane:
There is the Camera node (below Input in the Add menu). The Camera node has an "Octane Front Projection" output which when connected to the projection input of an image node ...

I am using Octane for Blender and I am trying to solve the following problem:
In Blender I need a backplate image (a jpg, not an environment image) in the Octane preview in the viewport to exactly match the Blender render border while in active camera view (numpad 0). In Blender Cycles you can ...