I think it could be considered an Easter Egg. Who says they have to be unlocked? The hidden Amazons in Rome: Total War don't have to be unlocked, and they can be considered an Easter Egg. The Egyptian Protectorate message is one too, in my opninion. "All your base are belong to us" is not a standard or historical message. And since it is not advertised or enumerated as a feature, I think it could be classified as hidden.

Wikipedia's article on Easter Eggs
In computing, Easter eggs are messages, graphics, sound effects, or an unusual change in program behaviour, that occur in a program in response to some undocumented set of commands, mouse clicks, keystrokes or other similar stimuli intended as a joke or to display program credits. A former use of the term Easter egg was to describe a message hidden in the object code of a program as a joke, intended to be found by persons disassembling or browsing the code.
So the ESRB is wrong for cracking down on mods, but only because that's the press this is getting. It's been mislabelled, and modifications can only stand to lose.

Stupid Rockstar.