There is no out of the box support for this use case, but in dm Server 2.0 I think you can use the new repository support to satisfy the requirement.
Let's assume there is an appserver dm Server instance configured with a repository chain consisting of a local repository and a hosted repository. In the appserver configuration, you can set a polling interval (in seconds using the <repo-name>.indexRefreshInterval property in the repository.properties file in the appserver's config directory - note we recently rebased our configuration mechanism on OSGi ConfigurationAdmin) which the appserver uses to synchronise its index with that of the hosted repository. The overhead of polling when there hasn't been an update is very small - essentially we send a HTTP get to the hosted repository and get back a 304 (NOT MODIFIED) response. So the polling interval can be set relatively small.
Then you need to write an application, which will run on the appserver and poll the repository mbean. When a new version of the WAR is added to the hosted repository and the appserver has polled and updated its index, the mbean will return the new version in the result of the call to the repository mbean. The application can then undeploy the old WAR and deploy the new WAR, using for example the deployer mbean.
There is one mbean per repository in the repository chain, so it is necessary to poll the mbean for the hosted repository. That mbean has name "com.springsource.server:type=Repository,name=host ed-repository". The attribute AllArtifactDescriptorSummaries returns short descriptions (type, name, and version) for all the artifacts in the repository.
It should be possible to get a proof of concept working with the latest nightly build of dm Server 2.0. I would then welcome any suggestions you might have for functional improvements if there are any major limitations in this approach.
How does that sound?