Thanks to the hint with the multiple galleries plugin from Niall Campbell and thanks to this question How to Add a Custom Colum on Thickbox Media Gallery Tab? (where I got the hook admin_head-media-upload-popup
from), I was able to complete the task.
I've added an option to add a style
attribute to the gallery shortcode.

Here is the complete code:
add_action( 'admin_head-media-upload-popup', 'wpse_53803_script_enqueuer' );
function wpse_53803_script_enqueuer()
{
if( $_GET['tab'] == 'gallery' )
{
?>
<script type="text/javascript">
jQuery(document).ready( function($) {
// append the table row
$('#gallery-settings table#basic tbody').append('<tr><th scope="row" class="label"><label><span class="alignleft">Style:</span></label></th><td class="field"><select id="style" name="style"><option value="standard">Standard</option><option value="slideshow">Slideshow</option></select></td></tr>');
// set our vars
var $style = '', $is_update = false;
// Select parent editor, read existing gallery data
w = wpgallery.getWin();
editor = w.tinymce.EditorManager.activeEditor;
if (editor !== null) {
gal = editor.selection.getNode();
if (editor.dom.hasClass(gal, 'wpGallery')) {
$style = editor.dom.getAttrib(gal, 'title').match(/style=['"]([^'"]+)['"]/i);
var $is_update = true;
if ($style != null) {
$style = $style[1];
$('table#basic #style').find('option[value="' + $style + '"]').attr('selected','selected');
}
} else {
$('#insert-gallery').show();
$('#update-gallery').hide();
}
}
// remove standard onmousedown action
$('#insert-gallery').attr('onmousedown', '');
// Insert or update the actual shortcode
$('#update-gallery, #insert-gallery, #save-all').mousedown(function() {
var $styleAdd = '';
if (editor !== null)
var orig_gallery = editor.dom.decode(editor.dom.getAttrib(gal, 'title'));
else
var orig_gallery = '';
// Check which which style is selected
if($('table#basic #style').val() != 'standard') {
$styleAdd = ' style="slideshow"';
}
if ($(this).attr('id') == 'insert-gallery') {
w.send_to_editor('[gallery' + wpgallery.getSettings() + $styleAdd + ']');
}
// Update existing shortcode
if ($is_update) {
if ($styleAdd != '' && orig_gallery.indexOf(' style=') == -1)
editor.dom.setAttrib(gal, 'title', orig_gallery + $styleAdd);
else if (orig_gallery.indexOf(' style=') != -1)
editor.dom.setAttrib(gal, 'title', orig_gallery.replace(' style="slideshow"', $styleAdd));
else
editor.dom.setAttrib(gal, 'title', orig_gallery.replace(' style="slideshow"', ''));
}
});
});
</script>
<?php
}
}
It adds style="slideshow"
if the slideshow style is selected, otherwise it doesn't add anything. And it recognizes the set style if you update the gallery, so that the right option is selected.
Thank you!
There is no way to alter the gallery settings.
is just wrong. I have done it, but dumped the code =(~/wp-admin/*.php
file for that request and search forapply_filters/do_action
.