Question: I want to make a link that goes to wp-login.php for whatever domain I'm at now. Something like:

```
<a href="http://<script>document.write(document.domain);</script>/wp-login.php">Login</a>

```

But I don't know how to write / format that properly.

Answer: 
Write a small plugin, use the follow example source to add a item in the list of the left sidebar in the media manager inside the overlay popup.

Result of the source below:
![enter image description here](https://i.stack.imgur.com/D1aLT.png)

```
add_action( 'admin_footer-post-new.php', 'wpse_78881_script' );
add_action( 'admin_footer-post.php', 'wpse_78881_script' );
function wpse_78881_script() {
    ?>
    <script type="text/javascript">
        jQuery(window).on('load', function() {
            var media   = window.wp.media,  
            Attachment  = media.model.Attachment,
            Attachments = media.model.Attachments,
            Query       = media.model.Query,
            l10n        = media.view.l10n = typeof _wpMediaViewsL10n === 'undefined' ? {} : _wpMediaViewsL10n,
            My_new_item = undefined,
            Item_string = 'New Item!';

            jQuery(document).on( 'click', '.insert-media', function( event ) {
                var workflow = wp.media.editor.get();
                var options  = workflow.options;

                if ( undefined == My_new_item ) {
                    // see for wp.media.view.RouterItem also: wp-includes/js/media-views.js
                    My_new_item = new wp.media.view.RouterItem( _.extend( options, { text: Item_string } ) );
                    workflow.menu.view.views.set( '.media-menu', My_new_item, _.extend( options, { add: true } ) );
                }

            });
        });
    </script>
    <?php
}

```

* Init solution form [this WPSE Answer](https://wordpress.stackexchange.com/a/77744/170).
* [Link to draft plugin](https://gist.github.com/Fab1en/4586865) example to add a javascript menu into the WP3.5 Media Library popup
I do not have a full answer to your question, but here is a good start.
To customize the new Media Manager, you should study the javascript Backbone code in `wp-includes/js/media-views.js`. 
For exemple, here is a small plugin that adds a "Select All" button to the "Insert from URL" Toolbar :

`custom.php`:

```
add_action('admin_enqueue_scripts', 'custom_add_script');
function custom_add_script(){
    wp_enqueue_script('custom', plugins_url('custom.js', __FILE__), array('media-views'), false, true);
}

```

`custom.js`:

```
var originalToolbar = wp.media.view.Toolbar.Embed;
wp.media.view.Toolbar.Embed = originalToolbar.extend({
    // code modified from media-views.js, l 2500
    initialize: function() {

        this.options.items = _.defaults( this.options.items || {}, {
            // keep the original button
            select: {
                style:    'primary',
                text:     wp.media.view.l10n.insertIntoPost,
                priority: 80,
                click:    this.clickSelect,
                requires: false
            },
            // and add a new one
            selectAll: {
                text: 'Select All',
                style: 'primary',
                priority: 80,
                requires: false,
                click: this.selectAll
            }
        });

        wp.media.view.Toolbar.Select.prototype.initialize.apply( this, arguments );
    },

    selectAll: function(){
        console.log('select All');
    }
});

```

For the "Custom insert into post" button, I suggest to use the gallery shortcode instead. The UI already exist to select the desired images and insert the shortcode at the right place in tinymce. All you have to do is to write your own gallery shortcode format. 

Take a look at `gallery_shortcode` function in `wp-includes/media.php` and use the `post_gallery` filter.