Might be too late to help, but yes, you can scope friendly_id. From the documentation:
class Restaurant < ActiveRecord::Base
extend FriendlyId
belongs_to :city
friendly_id :name, :use => :scoped, :scope => :city
end
class City < ActiveRecord::Base
extend FriendlyId
has_many :restaurants
friendly_id :name, :use => :slugged
end
City.friendly.find("seattle").restaurants.friendly.find("joes-diner")
City.friendly.find("chicago").restaurants.friendly.find("joes-diner")