Update for Grails 2.2+

As of Grails 2.2-RC1 it is possible to simply add a bindable:true to the constraints section of the domain class to allow assignment in the constructor / findOrCreateWhere:

class MyDomain {
    static constraints = {
        // allow binding of "id" attribute (e.g. in constructor or url parameters)
        id bindable: true
    }
}