No, I didn't. I believe that would qualify as "something procedural at a very basic level." (Sigh.) I'll try it right now.
While I was awaiting help from the cavalry, I started thinking about my menus. They are high-resolution PSDs, square-pixeled, 16:9 aspect ratio. I assumed Encore would interpret them appropriately for DVD output, but it did cross my mind that they might be telling Encore I want to go out HD. Is this a possibilty? I wondered if I might have created an incompatibility between menus and assets that was preventing linkage.