Executing an action for each found host name (or other element in the search input response) isn't possible today. I like the idea and I think Watcher should have a compound action that based on a for loop construct executes sub actions that are configured with it.
Unfortunately I don't know of a Watcher only workaround to send multiple webhook requests based on the number of elements found in an input response.
Perhaps you can send the web hook request to Logstash that is configured with an http input and do the logic there? See this blog for more info: https://www.elastic.co/blog/introducing-logstash-input-http-plugin