"jr2017_loaded" is the answer to my question tonight. By setting the action to hook after wp is loaded, my additional sidebar will be listed after the parent theme sidebars. (in the back end)
Thanks John P Bloch!
How to register_sidebar() without messing up the order?
How do you register a sidebar in a plugin without screwing up the pre-existing registered sidebars? (The ordering of the sidebar registrations, rather than the assigned ID, determines the sidebar data.)
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