Mary, you are good. Wouldn't have imagined student view of gradebook would be required for me to grade an activity, but sure enough: enable grade view, and now "Grade" shows in forum setting. (Or new ones, anyway. Apparently once a forum has a response, grade can't be edited in it.)
Does the Gradebook have to be visible to students for this? We use a 3rd party gradebook, and Moodle GB calculations would be hard to make agree with the one we use. Would be nice to keep it hidden, but still use it for my own marking.
But thanks for the help!