I just took a peek at this plugin and would like to propose a couple of modifications for the most recent version:
  1. Allow either a single admin user or all users in an admin group (or both) to have the customer-login access
  2. Add a session variable if the current session is an admin-login to allow an admin-login to have potentially different behavior

I'd be happy to implement these changes; I just didn't want to step on anyone's feet.