Hi Greg,
I remember seeing similar requests before for different activities, but this is not implemented. So, yes, if such indicator is needed, then it will need to be done manually.
You probably already know that, but if you do not use "Aggregate only non-empty grades" feature , then such grades will be treated as zeroes for calculation of the total. This aggregation method can be undesirable, and the display of such grade will be "-" (not visual).