XMLHttpRequest(..., returntype=<DOM | e4x | SAX | javascript | ?>)

Given that Firefox implements e4x and SAX (SAX is chrome:// only), and given that these could be useful for processing XMLHttpRequest responses, and given that it doesn't make sense to build a DOM and then convert to an e4x object or a SAX stream, I would like to pass a return type of DOM|e4x|SAX|javascript|??? to the XMLHttpRequest and just write an appropriate response handler. Is that too much to ask?