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 toyed with a custom field permalinks plugin, but as it's a meta box within a custom post type it doesn't seem to be picking it up. Is this possible?