This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Re-iterating the opinion of the other experienced developers: this is not an appropirate way to extend the capabilities of a JSP.
Not only is it pointless; interfaces are for allowing external code to deal with an object instance in a specific manner, and there is no external access to a JSP. But there are many supported means to extend JSP behavior: custom actions, EL functions and so on.