[Plugin: Custom Fields Permalink] Does it work with custom post types?

I have a custom post type called ‘portfolio’. Within this there is a meta box/custom field labelled ‘subtitle’. At the moment any post added to this custom type is displaying with the following permalink structure: http://www.example.com/portfolio/portfolio-title/

What I’d like to do is this: http://www.example.com/portfolio/portfolio-title/subtitle/

I’ve installed the plugin and it seems to be working ok, but it won’t add the ‘subtitle’ custom field to the permalink. Is this possible?

http://wordpress.org/extend/plugins/custom-fields-permalink/