The Revit API does not provide a way to extract all the details of a Revit decals, so if you wish to render decals in Octane, simply replace the decal in Revit with some geometry and apply the decal texturemap to that geometries material.
For example:
Important: In the properties box, assign a material that will convert to an Octane material that will have the decal texturemap loaded.
When you run the Octane plugin, select the extrusion created above, and setup it’s material in the MaterialsA set of attributes or parameters that describe surface characteristics. tab. The diffuse map scale should be changed from “float2” to “float”, and set the scale (to about 0.02).