Nice round look at current state of Ajax and WordPress:

http://www.wphardcore.com/2010/5-tips-for-using-ajax-in-wordpress/

Here’s the WordPress Trac discussion for implementation in WordPress 3.0:

http://core.trac.wordpress.org/ticket/12400

Otto:

There’s an example to add a quick button to TinyMCE at:

http://tinymce.moxiecode.com/examples/example_20.php

To do this in a WP plugin, you’d hook to mce_buttons_2 (or 3, or 4) to add your button to the list. Then you’d hook up a filter on tiny_mce_before_init to add your javascript code to the $initArray['setup']. That should be pretty much that, really.

Someone suggested to combine that a simple button api with the shortcode api to create a simple visual editor button api. Probably way to many use cases to cover, but it is an interesting concept. Some groundwork exists in the Super Emoticons code for WP Super Edit.

I needed to do two things to make it work:

On line 131 change is_int() to is_numeric():

if ( is_numeric( $blog_id ) ) {
switch_to_blog( $blog_id );
$current_plugins = apply_filters( ‘active_plugins’, get_option( ‘active_plugins’ ) );
}

Then in my .htaccess add the following:

RewriteRule ^custom-css.php /index.php [L,QSA]

Now works a treat.