The jboss-service.xml file in server/default/conf mentions the following:

<!-- The URLComparator can be used to specify a deployment ordering
 for deployments found in a scanned directory. The class specified
 must be an implementation of java.util.Comparator, it must be able
 to compare two URL objects, and it must have a no-arg constructor.
 Two deployment comparators are shipped with JBoss:
 - org.jboss.deployment.DeploymentSorter
 Sorts by file extension, as follows:
 "sar", "service.xml", "rar", "jar", "war", "wsr", "ear", "zip",
 "*"
 - org.jboss.deployment.scanner.PrefixDeploymentSorter
 If the name portion of the url begins with 1 or more digits, those
 digits are converted to an int (ignoring leading zeroes), and
 files are deployed in that order. Files that do not start with
 any digits will be deployed first, and they will be sorted by
 extension as above with DeploymentSorter.
 -->
 <attribute name="URLComparator">org.jboss.deployment.DeploymentSorter</attribute>

As per the comment in the jboss-service.xml file, you might have to write your own URLComparator, which implements the java.util.Comparator interface. The you can modify the following attribute as follows:

<attribute name="URLComparator">abc.xyz.myURLComparator</attribute>

See also http://wiki.jboss.org/wiki/Wiki.jsp?page=MainDeployerEnhancedSuffixOrder

Since 4.0.1 it can be controlled through conf/xmdesc/org.jboss.deployment.MainDeployer-xmbean

enhancedSuffixOrder attribute