Work on this is being carried out on a branch:
https://github.com/bstansberry/jboss-as/tree/deployment-scanner
John Bailey has gotten a big chunk of the core of it done, if folks want to experiment. We need to add in exploded deployment handling (which is largely a difference in how the content is registered with the deployment repository.)
At this point we're only going to go with the marker file approach; the "legacy" approach is not robust and resources are limited so the core AS dev team will only pursue it if resources become available or its clear there is major community demand for it. (Proof of community demand would be a solid, well tested patch. :) )