The GPL does not allow people to impose additional copyright restrictions on top of those that already exist in the terms and conditions of the GPL itself. However, the GPL does allow the original licensor to grant exceptions or "additional permissions" by waiving some of the terms and conditions imposed by the GPL.
Could these exceptions be written in such a way that they apply to only certain individuals, or if certain conditions are met?
Example: "green-eye privilege"
Consider if a program author added the following text to their copyright file:
As a special exception to the GPL, when a person with GREEN EYES modifies this program, they may distribute their modified version under ANY TERMS of their choosing (i.e. they are not required to distribute their version under the GPL).
Would this exception for green-eyed people (and others like it) be valid?
The exception might be accompanied by the usual non-obligation text:
If you modify this program, you may extend this exception to your version of the program, but you are not obligated to do so. If you do not wish to do so, delete the exception statement from your version.
But, in the case of the GPL at least, even if not stated, the non-obligation clause would still apply.