Monkey patch ActiveResource::Base.user so that it calls @user if user is callable, otherwise uses the default behavior.

activeresource_base_callable_user.rb

module ActiveResource
  class Base
    class << self
      # Monkey patch ActiveResource::Base.user a little. We want it to call
      # @user if @user is callable, otherwise use the default behavior.
      def user_with_lambda
        if defined?(@user) && @user.respond_to?(:call)
          @user.call(self)
        else
          user_without_lambda # default behavior
        end
      end
      alias_method_chain :user, :lambda
    end
  end
end