Well yes, but in this case it wasn't just them getting information, CLAMP got to contribute into who and what some of the characters were.
"While developing the character designs for Lelouch Lamperouge, the protagonist of the series, Clamp had initially conceived of his hair color as being white. Ageha Ohkawa, head writer at Clamp, said she had visualized him as being a character to which "everyone" could relate to as being "cool", literally, a "beauty""
Of course, their input wasn't universal. If I recall correctly, one of Mokona's favorite designs that she created for it was Clovis, and she was sad when he was killed in episode 2 (much earlier than was originally intended) which she didn't know was going to happen.