Using with_scope with more than one model

I've seen a lot of usage of with_scope since my last post, and everywhere I
see code like this:

Mail.with_scope(:find=>{:conditions=>"user_id = ..."}) do
  count = Mail.count
end

Here, both the model used for scoping, and the model inside the scope is the
same.

I'm trying to do something like this:

class User<ActiveRecord::Base
  def among_friends
    User.with_scope(:find => {:conditions => friends_scope_condition}) {yield}
  end
end

so that I can do stuff like:
    #Find friends' quotes
                current_user.among_friends do
      @quotes_by_friends = Quote.find(:all, :order=>'created_at DESC')
    end

Unfortunately, this doesn't work, and to correct the code, I need to use
Quote.with_scope in the among_friends definition instead.

This defeats the purpose; which was to increase flexibility.

Would appreciate any ideas.