Just FTR, this could be achieved simply by introducing (and redirecting) to new MailType, one that might pick up _real_ Velocity template from somewhere...

See the (default) MailTypes used currently by nexus, but these might be contributed by Nexus itself:
https://github.com/sonatype/sisu-mailer/blob/master/src/main/java/org/sonatype/micromailer/imp/DefaultMailType.java
https://github.com/sonatype/sisu-mailer/blob/master/src/main/java/org/sonatype/micromailer/imp/HtmlMailType.java