Call it compile_methods! and do the same on AM.

actionmailer/lib/action_mailer/railtie.rb

@@ -18,6 +18,10 @@ module ActionMailer
       options.javascripts_dir ||= paths.public.javascripts.to_a.first
       options.stylesheets_dir ||= paths.public.stylesheets.to_a.first
 
+      # make sure readers methods get compiled
+      options.asset_path           ||= nil
+      options.asset_host           ||= nil
+
       ActiveSupport.on_load(:action_mailer) do
         include AbstractController::UrlFor
         extend ::AbstractController::Railties::RoutesHelpers.with(app.routes)
@@ -25,5 +29,11 @@ module ActionMailer
         options.each { |k,v| send("#{k}=", v) }
       end
     end
+
+    initializer "action_mailer.compile_config_methods" do
+      ActiveSupport.on_load(:action_mailer) do
+        config.compile_methods! if config.respond_to?(:compile_methods!)
+      end
+    end
   end
 end

actionpack/lib/action_controller/railtie.rb

@@ -38,9 +38,9 @@ module ActionController
       end
     end
 
-    config.after_initialize do
+    initializer "action_controller.compile_config_methods" do
       ActiveSupport.on_load(:action_controller) do
-        config.crystalize! if config.respond_to?(:crystalize!)
+        config.compile_methods! if config.respond_to?(:compile_methods!)
       end
     end
   end

activesupport/lib/active_support/configurable.rb

@@ -10,12 +10,12 @@ module ActiveSupport
     extend ActiveSupport::Concern
 
     class Configuration < ActiveSupport::InheritableOptions
-      def crystalize!
-        self.class.crystalize!(keys.reject {|key| respond_to?(key)})
+      def compile_methods!
+        self.class.compile_methods!(keys.reject {|key| respond_to?(key)})
       end
 
       # compiles reader methods so we don't have to go through method_missing
-      def self.crystalize!(keys)
+      def self.compile_methods!(keys)
         keys.each do |key|
           class_eval <<-RUBY, __FILE__, __LINE__ + 1
             def #{key}; _get(#{key.inspect}); end

activesupport/test/configurable_test.rb

@@ -49,7 +49,7 @@ class ConfigurableActiveSupport < ActiveSupport::TestCase
     assert !child.config.respond_to?(:bar)
     assert !child.new.config.respond_to?(:bar)
 
-    parent.config.crystalize!
+    parent.config.compile_methods!
     assert_equal :foo, parent.config.bar
     assert_equal :foo, child.new.config.bar