[wp-hackers] wpAjaxResponse and XML elements

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Is there some way using the built in wp-ajax-response script/wpAjaxResponse 
to send XML Elements that share the same name? Or is there any other 
recommended way than going with JSON? In some cases this isn't possible 
(external APIs providing only XML as response for example).

To better illustrate my problem with the callback function and the desired 
output: http://pastie.org/2452302

Thanks, 
K.