I think that the colorspace gamut has to match the reference colorspace gamut in the OCIO.config file, or match a “to-reference” transform.
The TIFF used for input in @gez’s Blender demo produced using dcraw has sRGB primaries assigned, and the corresponding data, per the man page. That would work with Rec709-referenced OCIO configs, as the color primaries in both specifications are the same.