I'm trying to do this as well. I've dug through the SWF source and the IOC bean docs for Spring.
The WebFlow FlowRegistry does not provide any method for unregistering a flow. It would seem the intention is that the registry is completely re-instantiated in the ApplicationContext if flow definitions need to be updated. Can any developers from the SWF project comment on this?
The best approach I have come up with so far is to extend the FlowRegistryFactoryBean with additional methods to retrieve flow definitions from an external source. I have not however figured out how to plug this into the IOC container so that, for instance, a new FlowRegistry bean could be instantiated on-request. This approach would also require incoming requests to be queued in the FlowHandlerMapping while the FlowRegistry is rebuilt; there does not appear to be a provision for this either.