Mechanism to disable keys

[ghost]

This was mentioned at tex-stackexchange.com (my username there is RobtA, not the same RobtA as GitHub).

I am currently hacking some fontspec commands so that I can intercept user requests for certain font features. For example, in a document with [gray]{xcolor} I need to disallow Color and Opacity, because these circumvent xcolor.

Currently I modify commands such as addfontfeature, with code that scans the feature for the forbidden words.

The desired modification to fontspec would be to have a package option, or setting, that incorporates this capability directly in the package.

EDIT: Earlier today, over at tex.SE, egreg posted something that works (which I have slightly modified to include Opacity). Works great. All it needs is a user option, to turn it on or off. Link:

[http://tex.stackexchange.com/questions/354127/l3-update-bug-from-earlier-today/354277?noredirect=1#comment872031_354277]