Register IP address of anonymous sender.
Captcha for anonymous senders (optional).
Ability to type nickname for anonymous senders, and (optional) their email, just like in comments.
Ability to attach at least one photo (maybe more?) by anonymous senders.
Pretty smart plugin, but unfinished, and now abandoned, was http://wordpress.org/extend/plugins/post-from-site/.
Maybe you can implement some of it's functionality into your plugin?