The idea here is a small server-side process - perhaps with its own webserver, that can receive and respond to requests for data held in SDE. The requesting app would have to be an ArcGIS Server. There would be basic config to allow trusted connections from the ArcGIS Servers. I don't want this to blow up into a WMS/WFS issue - but the idea is to allow for 'occassionally connected' sessions from an ArcGIS Server without out the need to keep the pipe wide open at all times. So, the REST call could open the connection and send session information to the 'Connection Gateway' on the SDE server. The lightweight app on the server would time these out, manage caching timeouts, etc.