Just replying to myself in case anyone is interest in my current experimental solution. In addition to watching MDL-8902 I've currently changed line 756 in datalib.php so get_my_courses filters on the capability user:viewusergrades instead of course:viewcourse. It seems to fulfill our needs at the mometns but will wait and see if there are any knock on effects in further testing.
Alison
Roles and permissions
Creating a 'Preview' Role and My Courses Problem
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