Thanks for the offer, but I managed to do the changes by myself.
If others are interested, it happens in : wp-content/plugins/bridgedd/bridgedd_widgets.min.addon .
You’ll need to beautify this file (for example with https://universalindent.sourceforge.net/ , it also contains a PHP beautifier).
Then the main stuff is to change the SQL call from :
$sql = 'SELECT t.topic_id, t.topic_last_post_id, t.topic_last_poster_id, t.topic_last_post_time, t.topic_title, u.username, u.user_colour FROM '
. TOPICS_TABLE . ' t, ' . USERS_TABLE . ' u WHERE '
. sql_in_set('t.forum_id', ((!empty($wflag)) ? $guest_forums : $user_forums))
. ' AND u.user_id = t.topic_last_poster_id ORDER BY t.topic_last_post_time DESC LIMIT 0,' . $limit;
to :
$sql = 'SELECT t.topic_id, t.topic_last_post_id, t.topic_last_poster_id, t.topic_last_post_time, t.topic_title, u.username, u.user_colour FROM '
. TOPICS_TABLE . ' t, ' . USERS_TABLE . ' u WHERE '
. $db -> sql_in_set('t.forum_id', $Forums !== false ? $Forums : (!empty($wflag) ? $guest_forums : $user_forums))
. ' AND u.user_id = t.topic_last_poster_id ORDER BY t.topic_last_post_time DESC LIMIT 0,' . $limit;
With “$Forums” containing the comma separated list of Forums you want to display ; example : “41”, or “41, 13”, etc…