I second this feature request. I've encountered the same problem - an object
with variable transparency looks fake (too strong specular highlight) and there
is no way to make it variable as well. Colored reflection is also a physical
possibility (glaze of a different color than a substrate).

I posted this request as FS#263 on the bugtracker. It is a nice possibility for
an extension in next version (3.7.1 or something). The old syntax
specular <number>
could be retained for compatibility, and a new syntax with curly braces could
provide a full-featured specification, similar to pigment:

specular {
  specular | phong | other future algorithm names //type
  <scalar> | <color> | <pattern> | <function>
  roughness <number>
  [metallic]
  [copy_transparency]
  [optional transforms, warps, turbulence]
}

This would also migrate all highlight-related keywords in the same block, which
is more consistent with other blocks (such as reflection).

Does any of the official developers think this is an acceptable idea? It would
not be difficult to implement, just use Pigment_Function class that already
exists (yes, I looked at the code). I could take my time and write a patch for
this.

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