FEATURE: support registration of custom html by plugins

app/controllers/application_controller.rb

@@ -243,12 +243,20 @@ class ApplicationController < ActionController::Base
     end
 
     def custom_html_json
-      MultiJson.dump({
+      data = {
         top: SiteContent.content_for(:top),
         bottom: SiteContent.content_for(:bottom)
-      }.merge(
+      }
-        (SiteSetting.tos_accept_required && !current_user) ? {tos_signup_form_message: SiteContent.content_for(:tos_signup_form_message)} : {}
+
-      ))
+      if SiteSetting.tos_accept_required && !current_user
+        data[:tos_signup_form_message] = SiteContent.content_for(:tos_signup_form_message)
+      end
+
+      if DiscoursePluginRegistry.custom_html
+        data.merge! DiscoursePluginRegistry.custom_html
+      end
+
+      MultiJson.dump(data)
     end
 
     def render_json_error(obj)

lib/discourse_plugin_registry.rb

@@ -12,6 +12,7 @@ class DiscoursePluginRegistry
     attr_accessor :desktop_stylesheets
     attr_accessor :sass_variables
     attr_accessor :handlebars
+    attr_accessor :custom_html
 
     # Default accessor values
     def javascripts

lib/plugin/instance.rb

@@ -108,6 +108,11 @@ class Plugin::Instance
     @javascripts << js
   end
 
+  def register_custom_html(hash)
+    DiscoursePluginRegistry.custom_html ||= {}
+    DiscoursePluginRegistry.custom_html.merge!(hash)
+  end
+
   def register_asset(file, opts=nil)
     full_path = File.dirname(path) << "/assets/" << file
     assets << [full_path, opts]