With MC's database design, I don't think this is currently possible. The database would need a way to independently track each actor and have a rating field associated with each actor. The problem is that MC's "records" are of files. Let's call them movies. So a movie record has an actors field, but that field is multi-value because there are usually 2 or more actors in each movie. So an actor rating field, in a movie record, doesn't make much sense.
As you've found out, you can put ONE actor in the artist field, but that doesn't help if you really want to track many actors.
I believe you would need a database redesign and/or to have the JRiver team build this functionality. I don't think it's possible to do with the interface and tools that are presented to the end user. But maybe someone will surprise me!
Brian.