The params argument will be used as BODY when a post is issued.

Y.mojito.lib.REST.POST('your-url', 'BODY does here', {headers: {}, timeout: 12345}, myCallbackFunction);

Official documentation is here, and it does not cover the POST:

http://developer.yahoo.com/cocktails/mojito/docs/api_overview/mojito_rest_lib.html

Docs contributions are welcome!

P.S: REST Library in mojito is just a wrapper around Y.io, my recommendation is to use Y.io instead :)