You could always just code it so that dunks are only attempted 1 out of every X number of times once it falls below a certain tendency threshold even for open dunks, so that guys with a decent tendency will still dunk it in with no traffic-- because you don't want dunks so tied to tendency for capable dunkers that you have no control over when dunks go off.
So basically you could do something like:
Dunk Tendency 0-25: Rating affects dunk attempts by user regardless of how open.
Dunk Tendency 26-99: Rating applies only to dunk attempts in traffic.
Or you could have a new tendency called Open Court Dunks and apply the same threshold as above, so that the CPU uses the tendency directly but the user has control over whether to force an open court dunk or not except when the rating is below a certain threshold, and can only hope for a once in a blue moon dunk.
That dunk should give a Posterizer type momentum boost though just because of the rarity.