It seems like you could do something like set up the webhooks thing to use a url like
http://www.mydomain.com/misc.php?do=webhook. Then create a plugin using hook misc_start that checks for $_REQUEST['do'] == 'webhook' and saves the info to the database, maybe using the datastore. Then your forumblock code could just format the info from the datastore. It wouldn't be secure (anyone who knew the url ended with misc.php?do=webhook could update your data), but maybe that's not a big issue.