The gauntlet is dropped, and a worthy challenge . There seem to be a number of free IES file generators out there, but nothing directly disclosing the formulaic translation between a given light cone geometry and resulting ies file.
However ArchiCAD does have an explicit IES light source option, where a manufacturer's or other IES file is loaded by the ArchiCAD light object to generate the light cone. For this light type, If the IES data is then accessible to Octane, along with the light position and orientation, would this suffice for Octane to be able to generate a polygon of same position and orientation , using same ies data as a texture?
To clarify my motives - if I want a spotlight light coming from a direction where there is no geometry modelled, or where modelling anything would be counter-productive to workflow and intent (eg where the model is still schematic rather than realistic, and spot lighting is purely for atmospheric effect, and where there is a need to iteratively test, then adjust angle and characteristics of the light source on the fly until the right effect is created) .
There are frequent cases where a 'real' spotlight object would enter or even obstruct the desired camera view of the scene when this is undesirable and a distraction from the intent of the scene - diverting a lot of effort away from the desired outcome and into workarounds for hiding light objects where the light effect but not the object containing light emitting polygons needs to be visible
Octane Render for ArchiCAD 21 plugin v3.x | Octane Standalone 3.x
(2 x Nvidia GTX780 + 2x Nvidia 980 Ti) x 6MB VRAM | Intel I7 4470K | 16GB RAM | Windows 10