I have tried to create this using the resolved/unresolved status. If you mark something as unresolved, you can accept an answer from any post. In the back-end, this logs the answer post id and the date and time. This can then be used by a third party system to allocate points (we will be making something like this public on dnn.com eventually). The awarding and display of points itself is not in the forum module (and personally I feel it shouldn't be, it should be across all modules).