After weeks of wrestling with issues mostly related to adding and deleting extensions in the new phpBB 3.1 architecture, I finally got past them with a lot of support from the extension writers forum on phpbb.com. The Administration Control Panel user interface is now pretty much done, and the actual user interface part got done in just a morning of work. Fortunately I was able to plug in a lot of code from the old version once I had a framework working, which I got from examining the ACP board settings program. See attached screenshots. As you can see, Smartfeed now appears on the Extensions tab, which is generally where extensions should appear.
With the user interface mostly done I can now begin rewriting the program that actually generates the feed, formerly smartfeed.php. Due to the extension architecture, it will have a different URL.