I don’t think it is a good idea to make attribution a part of the licensing because
a) as Ambershee says it is probably unenforceable,
b) you just might turn off perspective clients (they might not want to to be forced to share the spotlight with you and if they sell the game to a publisher, it might be a turn off to them as well) and
c) probably clients would for one reason or another not end up do it.