MapCSS user defining variable Key/Value input

[nmak]

It is possible to let the user make a variable in JOSM using settings:

setting::key {
  type: string;
  label: tr("Key");
  default: "";
}

I was wondering if it is possible to use the users input to style lines.
I tried a few things including underneath, but it did not work. At this point I don't think it is possible. Can someone confirm?

way[setting("key")]{
  color: setting("linecolor");
}

If it doesn't work, it would be a nice feature to have.

Thanks in advance.