Hi snowcow,
to be able to render what you need, you would to follow a composite render workflow.
Here is some info:
https://docs.otoy.com/StandaloneH_STA/StandaloneManual.htm#StandaloneSTA/RenderPasses.htm
https://docs.otoy.com/StandaloneH_STA/StandaloneManual.htm#StandaloneSTA/ShadowCatcher.htm
https://docs.otoy.com/StandaloneH_STA/StandaloneManual.htm#StandaloneSTA/RenderLayerPasses.htm
https://docs.otoy.com/StandaloneH_STA/StandaloneManual.htm#StandaloneSTA/OctaneRenderCompositingExtensi.htm
Regards
Paride