If someone's face is within your reach there is no reason you shouldn't be able to figure a way to get back up. However, if you're talking about defending it in a friendly way (as in no thumbs embedded in eye sockets), nikyo can be implemented well from your back with a little practice.
This post half makes me want to start a James Randi style fund for martial arts.
$20.00 for the person who can eye gouge me while I have them in the mount before I get annoyed at them and pound them unconscious!
Must be willing to come to me because I'm really lazy.