If you commit to incurring the development overhead of an app server, you're probably best using the app-server's scripting/templating and permissioning conventions to write your pages.
Although I don't have deep app server experience, my intuition (and numerous developer buddies ) tell/s me that there would be so much overlap in functionality between a standard J2EE implementation and o/ACS as to render the exercise of combining them frustrating, and with little reward.
Both platforms require a significant learning investment to really reach fluency. I would hate to try to master both at the same time.
