I've added aria-modal to the ARIA 1.1 editor's draft:
http://rawgit.com/w3c/aria/master/aria/aria.html#aria-modal <http://rawgit.com/w3c/aria/master/aria/aria.html#aria-modal>
aria-modal (property)§ <file:///Users/jc/git/w3c/aria/aria/aria.html#aria-modal>
Indicates whether an element <file:///Users/jc/git/w3c/aria/aria/aria.html#dfn-element> is modal when displayed.
The aria-modal <file:///Users/jc/git/w3c/aria/aria/aria.html#aria-modal> attribute <file:///Users/jc/git/w3c/aria/aria/aria.html#dfn-attribute> is used to indicate that the presence of a "modal" element precludes usage of other content on the page. For example, when a modal dialog is displayed, it is expected that the user's interaction is limited to the contents of the dialog, until the modal dialog loses focus or is no longer displayed.
When a modal element is displayed, assistive technologies should navigate to the element unless focus has explicitly been set elsewhere. Assistive technologies may limit navigation to the modal element's contents. If focus moves to an element outside the modal element, assistive technologies should not limit navigation to to the modal element.
When a modal element is displayed, authors must ensure the interface can be controlled using only decendants of the modal element. In other words, if a modal dialog has a close button, the button should be a descendant of the dialog. When a modal element is displayed, authors should mark all other contents as inert (such as "inert subtrees" in HTML) if the ability to do so exists in the host language.