I'm having a little trouble finding and wrapping my head around this concept. I have a contrib module that defines a URL and page_callback with hook_menu. When I go to add a panel page and use that URL in the path portion of the add panel page form I get the message "That path is already in use. This system cannot override existing paths." as expected.

I am trying to override that URL with Panels functionality. I came across this article (http://deglos.com/blog/2010/10/06/overriding-urls-drupal-panel-pages) that seems to be what I need. Following and adapting that I still am no closer to where I need to be - a panel page I can enable and use for the predefined URL. Currently nothing is listed in the admin UI.

I have a custom module that invokes :
hook_ctools_plugin_api
hook_ctools_plugin_directory

I have a plugins directory with a tasks directory and in there a .inc file with the following hook:
hook_page_manager_task_tasks which returns an array that defines a hook menu alter function (among other keys). Within that menu alter there is a new page callback defined.

I have also compared to node_view.inc and the simpler, context-free poll.inc files within page_manager/plugins/tasks folder and those are fairly identical to my set up. Is there something I am missing to get it listed and functioning in the same way?

Let me know if you need anything else. Any nudges in the right direction would be extremely helpful. Thank you.