Is it okay to dry fire a flintlock without the frizzen closed?
No, you should never trip the lock without the frizzen closed. Without the frizzen there to absorb the energy of the cock's motion, the cock comes to an abrupt stop on the shoulder, and that inertia can bend the top jaw screw and is generally hard on the lock. You do not need a flint in the jaws to trip it safely, a piece of wood works too, but the frizzen must be closed.