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.
OK, after a quick scan of both the JSTL and Struts APIs (something you might consider) there is, as suspected, no specific method of including a Flash plugin.
It appears to me, however, that you could achieve the desired effect in JSTL using the c:import tag or in Struts using a Tile. I should point out that I haven't tried either of these approaches and the JSTL API doc for c:import is very terse.