[_ck_]

I assume this is just a typo in your example and not what you are really using:

add_filter('topic_pages', 'output_what_i_want, 11');

because the quote should close before the 11

add_filter('topic_pages', 'output_what_i_want', 11);

There’s nothing wrong with the action or filter triggers in bbpress (only that there are not enough of them or in the right places). The code for that is taken right out of wordpress and works fine. My dozens of plugins kinda prove they all work.