The latest version of Rhino (1.7R4) was released on 6/18/2012. According to the Mozilla docs on Date.toLocaleDateString, the locales and options arguments were added with the ECMAScript Internalization API, which looks like it came out in December 2012.
I think your best bet is to find a JavaScript library that can be included for your policy, and use that. See the Apigee JavaScript policy documentation for instructions on including JavaScript libraries.