the cxf file still loads in optimization here on v1.6.3... so not sure what the problem might be. Obviously 2052 patches of gray and all sides of near gray would be an preferable way to go. I regularly used it, but have to say that with large patch count rgb profiles, an optimized build using it was no improvement. With CMYK and N channel though, it was vital, initial builds sucked in the neutral axis on early versions.
But more to the point, I wonder why your cxf file is not loading, when you hit "load spot colors" in the optimization workflow, and you are directed to "load a CxF file", how is it not available?