Old Toad wrote:
This does not involve any plugins.
That's true, but to actually work it does require that the external editor app be able to process at least some of the file types the photo might be stored as, & for the Dock (or application) drop to work for the app to declare that capability to the OS. It also requires the editing app be able to resolve any ambiguities resulting from the possibility that the selected (or dragged) photo could represent more than one file (as in edited or original) or filetype (as in the several image formats that can contain embedded pre-rendered previews & such), possibly with the cooperation of iPhoto & the OS to run so called 'sanity checks.'
What I think is likely to happen with Photos is the same thing that the Photoshop family has done in the past with opening photos in the iPhoto Library package directly from the open dialog of those apps: the Media > Photos sidebar item will display the Photos library in the same way it now displays the iPhoto (& Photo Booth & any other package declared as a primary photo library) contents, using their built-in databases & such to organize the choices into whatever that library supports, like Events, Places, etc. for iPhoto. That will require the app to understand the Photo library structure, probably with the help of the 'open file' services the system provides & whatever services Photos provides to it as well.
At least, that's the way the open dialog of Preview works, & now shows me both the Photos & iPhoto libraries, each with their internal organizational schemes available & expandable under the library name.