A cookie is just a text string though, so Tacit can read the content of the cookie (possible to do in all browsers) and then set the cookie in the recipe. You could also make the cookie last forever unless they specifically blocked that on the server end.
As for setting a cookie it is done by using get_browser() in calibre to fetch the browser object and then set the cookie by creating a "cookie jar" and adding a cookie to it - Calibre uses the "mechanize" python browser. Please see this for further documentation and help:
http://wwwsearch.sourceforge.net/mechanize/