Made some progress

app/controllers/atom_controller.rb

@@ -0,0 +1,14 @@
+class AtomController < ApplicationController
+  before_filter :set_format
+
+  def user_stream
+    @account = Account.find_by!(id: params[:id], domain: nil)
+  end
+
+  private
+
+  def set_format
+    request.format = 'xml'
+    response.headers['Content-Type'] = 'application/atom+xml'
+  end
+end