Still investigating, but experiencing an ajax error. Not 100% certain this is a problem with Panels itself. The URL being retrieved is:

/panels/ajax/editor/select-content/panels_mini%3Amyminipanel/contentmain/page-content

Which is missing a slash -- the correct URL should be :

/panels/ajax/editor/select-content/panels_mini%3Amyminipanel/content/main/page-content