Support automatically resuming flow executions

I have a flow (book an appointment) that should be singleton per user; if the user gets distracted or otherwise doesn't complete the flow but later returns to the entry-point URL, instead of creating a new conversation, the flow should resume from the last-occupied state. Various people have apparently hacked around this by storing execution in the session and popping it if missing, but it would be helpful to be able to define a particular flow as singleton/auto-resume in the XML.