If the MailTemplateRegistry lookup returns null, it is possible that your jbpm.cfg.xml does not define any template, in which case MailTemplateRegistry is not injected into the environment.
In version 4.4, jbpm.default.cfg.xml no longer contains the predefined notification and reminder templates. Instead, it imports a separate resource, jbpm.mail.templates.xml. An empty jbpm.mail.templates.xml file could be the cause of the WireException.
Please attach your jBPM configuration file and any custom documents imported from it. As HuiSheng already pointed out, injecting a template into a custom mail producer is part of the test suite and should work with the adequate configuration.