As-yet-unknown means "currently unknown." In your sentence, Andrews does not yet know who the person who shot the footage is. However, he plans on suing him or her, as soon as he figures out who s/he is.
as-yet-unknown means that it is an unknown quantity (in the case above, that the person hasn't been identified). It also indicates that while that is the current state of things, that they are or will be working to identify the person.