Can a GFCI outlet intentionally and explicitly be used as a switch without violating code?
This is a great question. 

Of course the answer is no, but the NEC does not say you can't verbatim. You have to consider the intent of the code. To do that we have to look at Article 100 Definitions and what is a definition of a switch and what is the definition of a GFCI and what is the definition of a receptacle. Then you also have to reference NEC Articles 404 Switches and 406 Receptacles.

The point is switches and receptacles are two separate types of devices and it never states otherwise. You might argue that "it doesn't say I can't". But you have to look at Article 404.11 Circuit Breakers as Switches. It identifies that you can use a circuit breaker as a switch if it meets certain parameters.

So the "intent of the code" is that if it isn't defined in 404 that it can be used as a switch, then it can't be used as a switch. We could probably get this written into the code if we submitted it to the committee. Then it would get it written in within the next 6 to 9 years if they found it relevent. 

If this answer doesn't convince you then consider this question. If I have and incandescent lamp inline with some other devices can I use it as a switch by screwing and unscrewing it. The NEC doesn't say I can't.