How can I access '/node/%nid/webform/components' in panels?

I think you're looking for http://drupal.org/project/panels_everywhere, which has the ability to take any arbitrary page and turn it into a panel where the original content of that page becomes a pane within that panel.

Actually now that I'm more familiar with Panels, you can do this with any page just by adding the path to page manager and then overriding the page contents. There's a panel pane for "existing page content" or something like that that you can use to place the current page's content inside a panel.

The only caveat that is Webform-specific is that Webform uses the path "node/%webform_node/webform/components". You have to match the path exactly as defined in webform_menu() to override a path using page manager.

Closing this issue after lack of activity.