>To reply to the top level you may click on the top subject. This will bring you to the first message of the thread.

Not quite the "Here's a radio button that will let you do just that!" type of response I had hoped for, but it works (what I've been doing since the top subject became available as a link). But you stlll have to wait for another message to appear before you can post a reply, and what I really want is for the message text at the current level to be used in my reply back to the top level in the thread...sometimes.