I am using the event: 'core.viewforum_modify_topics_data', but frustratingly, the data does not easily tell me which forum is being displayed. It would seem to be sensible to add forum_id as a parameter.

I can workaround the issue with the code:
{{ //find the forum id... Unfortunately the event only gives us the topic ids, so we have to lookup.
if (sizeof($topic_list))
{
// Generate topic forum list...
$rowset = array();
$rowset = $event['rowset'];
$row = array();

foreach ($topic_list as $topic_id)
{
$row = $rowset[$topic_id];
if(!($row['topic_type'] == POST_ANNOUNCE || $row['topic_type'] == POST_GLOBAL))

{ $forum_id = $row['forum_id'] + 0; // only need one entry as we are only looking at one forum break; }

}
}}}

but it would be much tidier to have the value passed with the event (and it would work in the corner case where the event contains no topics).

Thinking a bit more... I think that there should be a policy that event data should always carry the identity of the instance that generated the event.