After something like six hours beating my head against a wall trying to add an extended field to a Tiddlywiki I was working on for a personal project, I finally stumbled across the cause. The key factor was my use of Monkey Pirate Tiddlywiki (MPTW) instead of using the plain vanilla version, as MPTW is slightly closer to what I think I will eventually need for my project.
I’m fairly sure the documentation for the plain vanilla version accurately reflects that one simply needs to add a div-span section to the EditTemplate and ViewTemplate tiddlers in ones wiki. However, in MPTW, the edit and view template functionality is contained in the MptwTheme document, under the Edit and View template sections respectively.
Maybe Google will turn up a hit for the next lost soul hunting this up and will possibly save a few hours.