Category mapping seems more preferable than picking as you post, but I'm thinking of how many times I would forget to pick the right board and have to repost.
Of course, you'd have the issue of dealing with multiple categories, which is the main reason why I wanted to not leave WordPress as a content source vs. using a forum software that has page elements to it (I moved over to Xenforo from IPS because of your bridge). That may be harder to tackle.
Is the method of picking which board to post in typing in the board number in the New Post panel?