You could "game" a trigger campaign by changing some unrelated field, or creating a new list, then triggering off the unrelated change or being added to the new list. In the trigger campaign you'd call your webhook in the flow.
For example, if you start collecting leads for a webinar before you have a webhook to pass data to the webinar provider, you can still set up a trigger campagin with a webhook call in the flow. To process the webhook for those leads:
Say you have your unprocessed leads in a list called "Registered 1"
--Created a new list (Registered 2).
--Set up a triggered Smart Campagin, triggered by "Added to List", with a flow of "Call Webhook"
--Opened up the list (Regstered 1)
--Chose "Select All", then "Lead Actions"/"Flow Actions"/Add to List, and specify "Registerred 2"
As the leads are added to Registered 2, the trigger will fire, and the webhook will be called.