This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
No, there isn't a way to use an <html:form> tag to refer to an outside URI. Think about it: If the action attribute doesn't refer to an action path, how is Struts going to associate a form bean with it? Even if it could, a form bean can't be passed to an outside link anyway.
The only solution I'm aware of is to do what you're already doing: Use the standard HTML tags.