I haven't tried to implement this, but I'm not sure how it would be useful, how are you wanting to use the choice? - what would they get access to after answering? - wouldn't this require a "correct" answer? - in which case isn't that what a quiz is? - a choice doesn't have a correct/incorrect answer - it's like a poll. - my guess is that the better option would be to modify activity locking to work with the choice module when a choice has been completed - not to modify the choice module to use a grade.
Dan