I need other success actions. Is it possible to extend this part of the form? I've already added custom fields and customer save actions. I would like to redirect to a specific URL, not a node in the tree. Is this possible? I see the Success Mode list items under Meta Data but it doesn't look like they are associated with anything (assuming it's hard coded in the framework)
You can actually do this with the existing Success Mode / Redirect. Instead of choosing or entering the path to a Sitecore item, use the Insert External Link option on the Success Page field.
EDIT:
This is done on the form itself. Be sure to use the Insert External Link button to set the value.