According to http://docs.oracle.com/javafx/2/deployment/self-contained-packaging.htm:
To add a custom UI to the MSI package, you can customize WiX template file used by JavaFX Packager using technique from 6.2.4. Consult WiX documentation for more details.
Assuming I figure out how to create a custom template file, how do I pass it into the JavaFX Maven Plugin?