How to assign merit points to not_the_current_user

Question

I am attempting to add Merit reputation system into a Rails app.

I have a votes model, and I want to award Merit points to the owners of objects that have been voted on.

#models/merit/point_rules.rb
score 2, on: 'votes#create', to: :owner_of_object_voted_on

and with

class Vote
  belongs_to :voter, class_name: 'User'
  belongs_to :votable, polymorphic: true

  def owner_of_object_voted_on
    case self.votable_type
    when 'blog_post'
      votable.owner
    end
  end
end

class BlogPost
  has_many :votes
  belongs_to :owner, class_name: 'User'
end

However, Merit is awarding points always to the current_user, rather than the object owner.

Does score to: take a symbol that represents a method on the vote model, and am I setting this up correctly?

The relevant source indicates that :action_user is being used, but I'm unclear where this is defined and whether it can be overwritten.