I think this is not possible. JBoss supports the deployment of EAR's as an unpacked directory structure. If you have a file myapp.ear, you may create a directory with the same name in the 'deploy' directory (including the .ear) extension.
This allows you to update a single file in the directory. However, the maindeployer watches the directory's 'last modified timestamp', not it's contents. So if you do a 'touch' to update the modified timestamp, the maindeployer will undeploy the whole ear and redeploy it.