orm.xml file should be possible to create from a wizard.
Its content should be like:
<?xml version="1.0" encoding="UTF-8"?>

<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm
http://java.sun.com/xml/ns/persistence/orm_1_0.xsd"  version="1.0">

</entity-mappings>


It should be placed in the conf directory, and a build-impl rule should put it
in the correct archive location:
META-INF/orm.xml

The AS 9 plugin currently will provide XML code completion for it.

Of course, a nice GUI editor would help, but I heard what you will say about
that already.