Automatic defect creation using attribute extension in RTC

I would like to automate the creation of defect in an external system (via cmd line/program) when a new defect is created in RTC : I would like the 'simpliest' solution and I was thinking of using the javascript attribute customization in RTC (instead of eclipse plugin extension)

Based on the article on jazz.net (https://jazz.net/library/article/1003) and the wiki (https://jazz.net/wiki/bin/view/Main/AttributeCustomization), my idea is :
- define a new custom attribute 'external ID' in the defect work item type (to store the reference id from the external system)
- use the Default Values script based customization to 'fork' the creation in the external system, something like:
dojo.provide("com.example.common.MyClass");
(function() {
dojo.declare("com.example.common.MyClass", null, {
    getDefaultValue: function(attribute, workItem, configuration) {
       var value = ... //Put here my 'external' call to create the 'external' defect
        return value;
    }
 });
})();

Do you think that's a possible/good way to do ?
Do you have a better solution in mind ?

I have also a concern regarding the access of the custom attributes from within the javascript: I'm not sure I will be able to set any value for the 'external ID' attribute using the javascript extension (https://jazz.net/wiki/bin/view/Main/AttributeCustomization#Accessing_built_in_attributes_of) ... Can anyone confirm ?

Thanks for your help