Specular exponent spectrum
Using independent settings for each colour channel is like using settings dependent on wavelength when doing spectral rendering. So the question could be re-formulated as:


  Is a material which changes its roughness with wavelengths physically plausible?


With some kind of nano-fabrication process you could build a material surface using some microfacets reflecting certain parts of spectrum and distributing each type of microfacets in a different way. So not only it is physically plausible, but it is also physically realisable. On the other hand, you normally don't encounter such behaviour in "common" materials.