How to Filter in the "Add Media Popup" to show only "unattached" Media

Can't deliver a working example but there is this relevant section in wp-includes/js/media-views.js on Line 3173:

media.view.AttachmentFilters.Uploaded = media.view.AttachmentFilters.extend({
    createFilters: function() {
        var type = this.model.get('type'),
            types = media.view.settings.mimeTypes,
            text;

        if ( types && type )
            text = types[ type ];

        this.filters = {
            all: {
                text:  text || l10n.allMediaItems,
                props: {
                    uploadedTo: null,
                    orderby: 'date',
                    order:   'DESC'
                },
                priority: 10
            },

            uploaded: {
                text:  l10n.uploadedToThisPost,
                props: {
                    uploadedTo: media.view.settings.post.id,
                    orderby: 'menuOrder',
                    order:   'ASC'
                },
                priority: 20
            }
        };
    }
});

One filter refers to uploadedTo: null and the other one to uploadedTo: media.view.settings.post.id. So this is equivalent to "All Attachments" and "attached to the current post".

When an attachment is unattached its uploadedTo value is 0. I just tried to create a new filter for that but it did not return any attachments at all (I do have unattached uploads in my db).

Line 581 of wp-includes/js/media-models.js shows how the filtering mechanism is constructed:

uploadedTo: function( attachment ) {
    var uploadedTo = this.props.get('uploadedTo');
        if ( _.isUndefined( uploadedTo ) )
            return true;

        return uploadedTo === attachment.get('uploadedTo');
    }

I had guessed that a zero value would pass _.isUndefined() but it does not. So I'm pretty much clueless here. But maybe this pushes you or somebody else into the direction. The media framework is still mostly undocumented.