Cheryl, that sort of change needs an issue for discussion before going very far.  I am dubious that it will work (but you can try to show otherwise).

1. overrideRootMenu does an insertion that I don't think you can do with a dict.
  mainmenu.menudefs[0][1].insert(6, closeItem)

2. The structure of menudefs is part of the extension interface.  (The file should say so.)  The test extension zzdummy is incomplete.  test_zzdummy has not yet been written, and the definition of ZaDummy.menudefs, currently commented out, needs to me made conditional on whether tests are being run (__init__.testing)