I don't recall seeing anything in the code that specifies the layout of plugs and receptacles. They need to be rated for the application, and different receptacles need to be used for different voltages.
My guess is that NEMA created and maintains the standardized plug and receptacle configurations used in North America. I know that they are called NEMA configurations...but perhaps a different organization now 'owns' the standard.
Using these configurations differently would not, IMHO, violate code. For example, if you had two _different_ 120/208V supplies (say one 400Hz and the other 60Hz) and wanted to make sure that equipment was not interchanged between these supplies, I would see no problem using the NEMA configuration specified for 277/480V for one of these supplies. Similarly, using the NEMA 240V receptacle configuration as the 12V configuration for your particular facility seems acceptable to me. For residential use, it would probably make sense to use an even less common receptacle, for example the 240V locking, or some suitable European receptacle that has a UL listing, but then you have to deal with cost issues.
I would be far more concerned with the use of a wiring device rated for _AC_ loads on _DC_ loads. One aspect of rating things like switches and receptacles is interruption capacity. A 20A DC load is much harder to interrupt (arcing and such) than a 20A AC load.
-Jon
