After fixing various issues with the project setup itself and multiple cleanups/recompiles, it seems that the following two approaches are both OK
serviceClass(com.somepackage.ServiceClass, '/WEB-INF/constructor-arg-xml-file.xml') {}
and
serviceClass(com.somepackage.ServiceClass) { bean ->
bean.constructorArgs = [
'/WEB-INF/constructor-arg-xml-file.xml'
]
}