For road closures that are in progress or set to start soon your best choice is to post on the Benelux Slack team: http://bit.ly/waze-belux
The Waze forum is in use for less urgent requests:
Luxembourg: https://www.waze.com/forum/viewforum.php?f=386
Belgium: https://www.waze.com/forum/viewforum.php?f=718
In Belgium non-urgent requests are often also handled on Slack.
Slack is also the platform to communicate with the Benelux community on various other topics.
