The 4.4 XSD does not allow the "name" attribute for <object /> or <ref /> elements which means that they cannot be used for HQL activity parameters which require a name.
I have noted that if you just ignore the XSD and specify a name anyway it does work (org.jbpm.pvm.internal.wire.descriptor.AbstractDescriptor defines a field "name").
Also it's worth pointing out that neither the user guide or examples provide an example on an HQL activity with a parameter from the execution context.