My vision is more simple, a "standard" render region, you select a region with alt+b and this region is exported to octane.
i think it need a fov and camera shift conversion to keep the same lens distortion with the render region height/width a kind of wysiwyg without any display object modification.
edit: that's i already do manualy in octane, to render each individual part of a scene and keep the same lens distortion of the overall view.
edit2: and obviously it need to automaticaly disable vignetting.
i7 8700k / 16Gib / GTX980Ti 6GiB / win10x64 1803 / Blender 2.78a / drivers 416.34 + Octane 4.0 Standalone / OctaneBlender