I work with accessibility experts and disabled users. I did a cursory look at WordPress's posting interface and it looks like two things can be done right off the bat to improve the accessibility of WP's backend.
Looking at post-new.php, specifically:
1) Add a label tag associated with the post title:
<label for="title">Enter / Edit your Post Title</label>
<input id="title" type="text" autocomplete="off" value="" tabindex="1" size="30" name="post_title"/>
A style can then be used to position the label outside the viewport for design considerations, but it's still in the DOM and accessible.
2) If adding a label tag is not kosher, then a meaningful title attribute can be used with the input for the post title:
<input id="title" type="text" title="Title of New Post" autocomplete="off" value="" tabindex="1" size="30" name="post_title"/>
I know JAWS will recognize the title attribute when in forms mode, relaying the input's purpose to the user.
3) The TinyMCE editor has some useful keyboard shortcuts that are not made known to novice WordPress users: Alt+Shift+Q (Toolbar), Alt+Shift+Z (Editor)
An internal anchor could be placed near the top of the post-new.php body, acting like a skipnav.
'Use the Keyboard shortcut Alt+Shift+Z to jump into the editor's body. Use Alt+Shift+Q to navigate to the editor toolbar.'
Style it with CSS to position off the viewport.
These edits would greatly enhance the accessibility of WordPress as a publishing platform for those using screen readers. Thoughts?