[TeppoM]

Thanks for the thread. Based on this I managed to develop a solution that does not require hassling with the template files. Just add this to function.php in your theme:

/**
* Add @mentionname after bbpress forum author details
*/

add_action( 'bbp_theme_after_reply_author_details', 'mentionname_to_bbpress' );
function mentionname_to_bbpress () {
$user = get_userdata( bbp_get_reply_author_id() );
if ( !empty( $user->user_nicename ) ) {
$user_nicename = $user->user_nicename;
echo "@".$user_nicename;
}
}