[Natanael_L]

I'd be XORing against some existing strings in the code of the same length to obfuscate the content, with some hidden method to invoke the reverse XOR to regenerate this challenge text string.

[ultramancool]

Sure, hiding it as a basic string is easy. But hiding it in a way that a simple code review won't catch is probably a lot harder.

[Natanael_L]

I think some array manipulation could do it if you're clever enough and don't make it obvious where all of the inputs comes from. So you'd make some particular parameters regenerate the string, and it wouldn't obviously stand out from the normal behavior.

[Magnets]

That sounds very difficult to hide