To simplify usage of sprints and increase user visibility, I suggest to integrate the sprint UI this extension provides more into the regular project UI.
I.e. the workboard could be replaced with the sprintboard, and the burndown chart could be an item in the project navigation.
Currently projects and sprints look almost identical, up to the point where a user might be confused by looking at a sprint project but missing the sprint UI.
Phabricator now has a Profile Menu mechanism for Projects. Could that be used? Is it going to be used?
see https://gerrit.wikimedia.org/r/#/c/270484/. A lot of former sprint stuff (points, workboard cards with points, column point totals) is part of phabricator now.
Note that the profile menu customization occurs only in the sprint application context. (accessed through /project/sprint)
See example here: http://phab10.wmflabs.org/project/sprint/profile/35/