I mean, without pole pieces, it's just disgusting. But even with, you're limited to a pressure of the Maxwell Stress, B^2 / (2 mu_0). A 1T magnet is equivalent to about 4 atm, if the field is entirely on one face and zero on the other.
Much better off using a servo, gearing and crank. Needs far less magnet (low torque motor), the magnet goes through many cycles as the plunger is pushed, you get all the mechanical advantage of the gear train and (about) as much power as the motor is capable of, without quirky or custom magnetic designs.
(Even if direct drive is stipulated, it's still better to use a transverse-field linear motor over a copper rod: the entire surface area of the rod can produce a shear pressure, rather than just the cross-sectional area of a magnet the same size. Well, fine, it works the same with a stack of properly magnetized ring magnets on a steel shaft, doesn't need to be an induction machine. But yeah, area or speed is the key to power.)
Tim