Chris: based on my findings you can currently either remove that hibernate-validator-legacy.jar which will put it back to work, or do something like:
<?xml version="1.0" encoding="UTF-8"?>
<classloading-domain xmlns="urn:jboss:classloading-domain:1.0" name="jboss.j2ee:extension=LoaderRepository,service=EARDeployment,url='myapp.ear'" parent-domain="DefaultDomain">
<parent-policy>
<before-filter>
<javabean xmlns="urn:jboss:javabean:2.0" class="org.jboss.classloader.plugins.filter.NegatingClassFilter">
<constructor>
<parameter>
<javabean xmlns="urn:jboss:javabean:2.0" class="org.jboss.classloader.spi.filter.RecursivePackageClassFilter">
<constructor>
<parameter>org.hibernate.validator</parameter>
</constructor>
</javabean>
</parameter>
</constructor>
</javabean-->
</before-filter>
<after-filter>
<javabean xmlns="urn:jboss:javabean:2.0" class="org.jboss.classloader.plugins.filter.NegatingClassFilter">
<constructor>
<parameter>
<javabean xmlns="urn:jboss:javabean:2.0" class="org.jboss.classloader.spi.filter.RecursivePackageClassFilter">
<constructor>
<parameter>org.hibernate.validator</parameter>
</constructor>
</javabean>
</parameter>
</constructor>
</javabean>
</after-filter>
</parent-policy>
</classloading-domain>
where you'll have to replace the domain name with the one valid for your case (you can find it via jmx-console for example).
But that will introduce the currently discussed change in classloadin behavior - plain war is parent-first=false while with jboss-classloading-domain.xml it will change to parent-first=true (and it's not that simple to switch it back while preserving the required package filters) which may (but maybe it won't) cause you some troubles.