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Justin, as you say, I think your patch is entirely reasonable as an interim step, as eventually doing a broader improvement on the preferences dialog as suggested in #24781 makes sense. 
My reworked version used Combobox in similar ways; I think we can safely do away with the wrapper class and just use the ttk widget directly in the dialog (as the widget already handles the dynamic changes to the list, which the old tk_optionMenu didn't)