actually, you could make "custom nk" files... Tinman, the developer of the (seemingly, but not really dead) renderlight renderer did that. His nks have the ending ".dat", though. He costumized original nks or calculated them of other datas, with his own input values... His dispersion doesn't use caughy but sellmeyer, for example
It could simply be tabulated spectrum tags, separated in a file, to simply use them again, and again and again, and...
Here's the HP of renderlight, if you're interested. (As I said, the project is very slow, as he suddenly has to do so much, it appears...)