IF and only IF you can get to one or more of the water lines to the shower (optimally the actual line going to the shower head itself). You could use the temperature change in the pipe to indicate activity; with some lag after the water has been turned off which might invalidate this approach for your needs.
See the Senasys link in this thread for simple & reliable pipe mounted temperature switches. Then you'd need something connected to that switch monitored by HE per this discussion.
