The class org.apache.velocity.runtime.resource.ResourceFactory provides
no mechanism for allowing sub-classes of Template or ContentResources
to be returned.
Since ResourceManagerImpl makes a call to ResourceFactory.getResource(),
the only way to override the behavior is to subclass ResourceManagerImpl
and cut-paste-then-modify the code for loadResource().
At the very least, moving the ResourceFactory.getResource() call into
a separate, protected method in ResourceManagerImpl would greatly simplify
sub-classing Templates and/or ContentResources.