[_ck_]

It might be possible to create the MediaWiki cookie from the bbPress side and visa versa.

From the mediawiki side, you could do something as simple as

require('./bb-load.php');

wp_set_auth_cookie( (int) $user_id,0 );

Which will create the bbPress cookie. Where $user_id is the user’s bbPress id. Since you are using the same user table, it should be available?