[Plugin: MailChimp List Subscribe Form] Hook to filter widget title

I’m working on a website where I need to filter all the widget titles as they appear. (I’m adding spans to create an effect where each word has a different colour.) I’ve managed to make this work by adding the filter to every particular plugin that has a widget:

add_filter('widget_title', 'add_title_spans');
add_filter('link_category', 'add_title_spans');
add_filter('twitter-widget-title', 'add_title_spans');
add_filter('mc_header_content', 'add_title_spans');

However, I can’t figure out what to use for the MailChimp subscribe widget—mc_header_content doesn’t work. I’m pretty sure I’m just missing something—how do I go about filtering the title of the widget?

Thanks!

http://wordpress.org/extend/plugins/mailchimp/