On the hand-thrown end of things, you can send XML data inside a POST, pull it off the request in a servlet, then parse it any way you like. I just implemented an interface like that for a client with very limited scripting ability. Since it's a normal POST I can test it from a static HTML page with XML in an edit box. Dead simple, not to any particular standard except maybe REST architecture.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi