Can I host & run Easyling as an appliance, on my network, behind firewall?

Yes, it's possible, using AppScale

What is AppScale?

AppScale is a Google App Engine clone that replicates each service. It usees Cassandra for BigDB, Memcached for caching,NGinx and HAProxy for serving, own fault tolerance and elasticity layer, Celery and Rabbitmq for tasking, and refactored App Engine SDKs for the app servers, among others.