to answer my own question:
It seems I had the wrong idea about how the FileTransform worked.
From Configs/nuke-default/make.py :
create the colorspace, then appy using oiio.ImageBufAlgo.colorconvert

cs = OCIO.ColorSpace(name='sRGB')
cs.setDescription("Standard RGB Display Space")
cs.setBitDepth(OCIO.Constants.BIT_DEPTH_F32)
cs.setAllocation(OCIO.Constants.ALLOCATION_UNIFORM)
cs.setAllocationVars([RANGE[0], RANGE[1]])

t = OCIO.FileTransform('srgb.spi1d', interpolation=OCIO.Constants.INTERP_LINEAR)
cs.setTransform(t, OCIO.Constants.COLORSPACE_DIR_TO_REFERENCE)
config.addColorSpace(cs)