by Enrique Castro.
Hi:
I'm just working on this. My university is upgrading to Moodle 3.1 this July and we want to keep the old interface, making the new one optional.
Unfortunately, I have this working as part of a local_ulpgcassign plugin that add some other functions that may make no sense in you installation.
What I have done is to add new site-level setting to allow admins to specify which submissions plugins will use the new interface. I mean, we use a lot of assign instance as a way to deliver assessment for offline assignments. We have some custom assignment submissions that do not imply student submissions. The new grading panel just doesn't makes sense with offline o no-submission assignments. I as admin can leave those opt-out so the interface only shows the link to the old grading page.
In addition to settings.php this involves a simple customization of colums col_garde and col_userid in gradingtable.php
If interested, I can make a patch against moodle 3.1 without other ULPGC customizations, but only after July 20. I'm quite busy before that date.