Cubemaps for VR? Yes, you can render this kind of scenes. In the render target select the panoramic camera and set the camera type to "Cube Map", set the Fov angles to 360x180, enable the "Keep Upright" option and set the Stereo Output to "Side by Side". Also you need to set the camera resolution to 18432x1536.
Currently I don't have any reference about the upgrade price of the plugins for Octane 3. I think the the current 2.2 plugin could be sold at reduced price until the Octane 3 plugin release, so the price of the current 2.2 plugin plus the upgrade price to 3.0 could be near the same or even less than the full Octane 3 plugin license at normal price. But anyway it is too early to speak about prices and release dates
-Juanjo