You might try this work around to edit the gallery admin pages via the template, as a similar process has been used in a couple of cases involving other solutions.
Define a second site in DW one level deeper than the primary site definition. DW will object to having two sites defined that involve the same folders, but just ignore the objection.
The second site will have the powergallery folder at the top of the DW files panel. The admin folder, the templates folder, and the others will then appear at the "root" of the second site. At that point you can edit the admin template and it should then prompt you to update the admin pages linked to the admin template. Save the files as necessary and return to the primary site.
Once back to the primary site, upload the updated admin files to your server.
This is the process that I used and it seemed to work fine.