According to the beta3c documentation, a <collection> element may contain attributes from the 'object' attribute group. But in the binding.xsd file delivered with beta3c, this attribute group is missing.
Validating my mapping file with binding.xsd consequently produces an error, since I have specified a custom factory method for a <collection>. But the jibx compiler accepts the factory attribute, and produces code that works fine. This leads me to assume that the documentation is indeed correct, and that the binding.xsd should be adapted accordingly.