I'm not sure I understand the situation fully... You say you want to put the calendar behind a login script that uses sessions... Do you mean that you want to display the calendar on the same page that you'd get after doing a login on your website? Or are you talking about using the session to allow the user to login to the event administration page without having to login to EPC also... (it's been talked about before).
If you're in the 1st situation described, there should be no problems... as long as the rest of your script/webpage doesn't use the same variables that EPC uses.
I currently use EPC in two similar contexts, one is on the front page of a login site (custom built) and the other is a block on a PostNuke site, and both use sessions to control the login.
If you're in the 2nd situation, let us know and we'll take a look at it.