Configure the "Entries" HTML Editor? I would like to configure the rich text editor in Entries->Singles content blocks like I do in my plugins and forms.  I know I can configure Redactor config within the /craft/config/redactor directory, but that does not impact the editor within //admin/entries/.  

Where is the config for the admin entries editor?
If I'm understanding you correctly, you've got a Rich Text field you're using in one of your Single's field layouts.

That Rich Text field will have settings (under Settings->Fields->(click on the Rich Text field you're using for that single)).  From there, you can select the specific craft/config/redactor/config.json config file you'd like that specific Rich Text field to use.