Hi Rickard,
At the moment if the Staff Manager role is given the 'moodle/site:viewreports' and 'report/completion:view' capabilities, and then is enrolled in the course in the Staff Manager role (not as a Learner, and they must be enrolled in the course merely having the Staff Manager role in the user context is not enough), then they will be able to see and use the Course Completion report for setting RPL.
Unfortunately as it stands they will be able to see every user enrolled on the course, not just their own direct reports.
Changing that would require code changes to report/completion/view.php and also to the completion_info class in lib/completionlib.php, specifically changes to the get_num_tracked_users and get_progress_all functions to detect if it is a Staff Manager that is viewing the report, and if so only display their direct reports.
May also need additional security checks on the report/completion/save_rpl.php AJAX script to ensure that a staff manager can only add RPL to their direct reports.
I can see other people wanting this functionality though, it does make sense if Staff Managers can set RPL on Plans they should also be able to do it in ordinary courses. I know there are changes to RPL being planned for 2.7 and 2.8, do you want me to file an enhancement request?