Directory deployment to instances other than the default server instance (basically the DAS) is, in fact, not supported.
Directory deployment is focused as a developer feature, designed to speed up the edit/build/deploy/run cycle by reducing the time required to build the app (no need to build the archive) and to deploy it (in directory deployment the app server does not need to expand the archive because there is no archive to expand, which is what it does with an archive deployment as opposed to a directory deployment).
The general thought is that deployment and redeployment of an app to an environment with multiple instances is very likely to be managed more carefully because that environment is likely to be a more controlled testing or production environment, not a development environment. And the administrators of such an environment will typically expect to work with an archive file (a WAR, JAR, etc.) for deployment and not a directory where a developer is actively working.
Of course this may not be the case for everyone, but it seems to be true in most cases we've encountered.
This is not what you wanted to hear, I'm sure, but I hope that helps explain the thinking behind this feature.
- Tim
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