from some web searches, it seems what I wanted to do isn't possible without
editing the registry...

in some cases, editing the registry will be denied.

perhaps just un-installing it (unchecking the checkbox in the Add-In List is
good enough)

but, if there are better suggestions, I'm still listening.

thanks.

Mark,
If you delete the AddIn then you can use this code I found on web sometime
ago (author name unknown so my apologies to him / her) to remove it from the
list.
May not be complete answer to you problem but hopefully give you some ideas.

Sub InvalidAddIns()
   Dim lCount As Long
   Dim sGoUpandDown As String
   'Turn display alerts off so user is not prompted to remove Addin from list
   With Application
       .DisplayAlerts = False
       Do
           'Get Count of all AddIns
           lCount = Application.AddIns.Count

           'Create string for SendKeys that will move up & down AddIn
Manager List
           'Any invalid AddIn listed will be removed
           sGoUpandDown = "{Up " & lCount & "}{DOWN " & lCount & "}"

           Application.SendKeys sGoUpandDown & "~", False
           Application.Dialogs(xlDialogAddinManager).Show

           'Continue process until all invalid AddIns are removed since
           'this code can only remove one at a time.
       Loop While lCount <> Application.AddIns.Count
       .DisplayAlerts = True
   End With
End Sub