main_site_domain.rb

class MainSiteDomain
  def self.matches?(request)
    if request.host == DOMAIN
      true
    else
      # custom domain
      false
    end
  end
end

routes.rb

constraints(MainSiteDomain) do 
  match "hello" => 'hello#say_hello'
end

constraints(OtherSiteDomian) do
  match "/foo/hello" => 'hello#say_hello'
end