Can I add an icon & function to the “Upload/Insert” toolbar at the top of the content editor?

Question

Is there a built-in filter to add a custom icon and function to the WordPress "Uploads/Insert" toolbar? This is the toolbar that's located just above the content editor. The existing icon I want to replicate is shown in the blue circle of the image below.

I'd like to add a custom icon that loads the "Add an Image" wizard and passes a special parameter to the upload function such that the attached images get a special meta attribute they would not ordinarily get when uploaded via the standard icon.

Update: With the help of tnorthcutt's answer below, I've managed to hook into the media icon filter. From here, I should be able to attach a jQuery click event to the image or href and trigger the opening of the media-upload.php, passing a parameter to process the uploads as special...

//Upload custom images
function addMediaIcon($initcontext)
{
    return $initcontext.
   ' <a id="myID" 
      href="javascript:;" 
      onmousedown="return false;" 
      onclick="return false;" 
      title="tooltip">
     <img 
      src="wp-admin/images/media-button-image.gif" 
      onclick="javascript:alert()" title="" />
    </a>';
}
add_filter('media_buttons_context', 'addMediaIcon');

Answer 1

I'm inclined to say that this is possible. For instance, the Gravity Forms plugin adds an image here. Start with looking at the media_buttons_context hook.

Answer 2

You can also find a good example of using the media_buttons_context filter on the following page. http://www.morgadinho.org/2009/10/16/wordpress-media-buttons/

I know this probably covers what you have already, but wanted to share the link all the same... ;)