Malcolm,

When building ICC profiles I've found it very helpful to use the InputTables of the AtoB0 to hold an intermediate conversion, for instance the a 1024 entry 1-d conversion from Cineon to video . The CLUT can then just hold the 3d portion of the conversion I found the match was much improved going that way without having to create a large 3d lut. It's really helps out if you are working with linear space source files in Photoshop.

It may be handy to specify that intermediate space to be placed in Add3GammaCurves()

So for example,
Source=Cineon
DST=Srgb_Film
Intermediate=video10

could be broken into 2 OCIO transformations
one transformation from Cineon to video10. (This transformation would be 1024 entries and placed into the 52nd byte of the Lut16 structure)
The next transformation would be the video10 to Srgb_Film, placed in the CLUT , at the end of the InputTables list.

It looks like that you are creating a gamma 1.0 table in that spot right now.

Let me know if that makes sense.

-Joseph