In the spirit of The Simplest Thing That Could Possibly Work: Why don't we just standardize encoding the XML infoset in HTML forms?
Here's how you could POST an Atom entry (compare to example in draft 07):
<form action="http://localhost/foo" method="POST">
<input type="hidden" name="/entry/@xmlns" value="http://www.w3.org/2005/Atom"/>
Title: <input type="text" name="/entry/title">
Link: <input type="text" name="/entry/link/@href"/>
Id: <input type="text" name="/entry/id"/>
Updated: <input type="text" name="/entry/updated">
Summary: <input type="text" name="/entry/summary">
<input type="submit" name="submit" value="Submit">
That form encodes as
The pattern is: Name your HTML form elements using XPath syntax.
(What if all Atom powered blogs supported POSTing of form data in addition to the Atom syndication format? I guess I could construct a website that exposes a form like this, and permits you to direct your entry to any blog service. Not sure how useful that would be, but it doesn't seem any less useful than enabling a rich client do it ;) ).
I'm just reiterating the point I made here some time ago: form data is a perfectly good hypermedia representation, and it has a ton of support already in place.