CASS is predicated on confirming your identity and is based on your passport number. So, to ride in the cockpit, you need to have your passport on you. If not, you should be able to "jumpseat" but will be required to occupy a passenger seat.

I don't have the official documentation, but that's my understanding. Depending on the gate agents, if they know they're will open seats in the back, they won't check my passport.