In Ellipsis, we have repurposed the "SEO keywords" field and set up our Canonical URL service to pull the URL string from there if it is populated. (domain . primary_site_section - SEO_keywords else Basic_Headline - First_Pub_Date.html) This allows us to customize more SEO-friendly URLs and increase traffic dramatically.
However, the "SEO Keywords" field is not ideal for this purpose because:
Please introduce a new field that is specifically to be used for the purposes of customizing a URL.