The GET object does send the request as POST, so this is not your problem. The object only sends key/values as form data, you need XML. Even if you set the header to application/xml, you cannot put your XML content behind withount the equal sign of the "Add POST data" key=value structure. I also didn't work to glue the content to the "Set custom header"
We had a similar problem here.
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So the closest you can get is setting the header to "Content-Type: application/xml" and setting the KEY of Add POST data to your XML, but it might be to complex. In the end you have what you want with an extra "=" behind ...
It would work with an extra infrastucture inbetween, a small PHP script on a server which receives the malformed request and send a repaired one to the Indesign Server or you pay someone coding an extension?