We have a new module for this:
http://httpupdate.cpanel.net/cpanels...nel/LogMeIn.pm
This will allow you to query a server for a URL that allows the client to log directly into cPanel. The URL is good for a single use and the session is destroyed once the URL is clicked on. There's a sample script after __END__ in the module which shows a simple way to redirect them, by printing the URL directly as the location to their browser.
This module is currently only available in EDGE builds but will make its way to other builds soon.