After thinking about this problem a bit, I'm fairly sure that it's impossible. Strap in, this is a long one.
You see, the reason a magnet attracts a piece of iron is that the magnetic field induces a bunch of little dipoles in the iron (more precisely, the magnet coerces each atom's intrinsic dipole moment to line up in the same direction, but the net result looks the same). But here lies the problem: the force exerted on a dipole by a magnetic field is not due to the strength of the field, but rather on its gradient (ie rate of change). Specifically,
$$\mathbf{F = \nabla (m \cdot B)}$$
where $\mathbf{F}$ is the force on the dipole, $\mathbf{m}$ is the dipole moment, and $\mathbf{B}$ is the magnetic field.
The force that the entire chunk of iron feels is just the sum of all the forces that each tiny constituent dipole feels. Therefore, when we see magnets attract iron, the fact that the field of the magnet fringes at the ends and becomes weaker is crucial, so that the gradient is non-zero. So while <PERSON> is correct that an infinite sheet of dipoles creates a constant magnetic field throughout all space, this field won't accelerate a piece of iron at all, let alone uniformly!
In case the analogy to electrostatics is still confusing you, the key difference here is that magnetic sources only come in dipoles (as far as we've encountered, at least). 780
So, the problem is fundamentally different from the case of a charged particle in an area with a uniform electric field, because electric charges are monopoles.
So, now knowing that a uniform magnetic field won't serve our purpose, the question becomes: can we come up with one that can? The answer appears to be no. To see how, we will take the simplest model for magnetization of iron, which posits that it will be directly proportional to the magnetic field strength $\mathbf{H}$ (we use $\mathbf{H}$ instead of $\mathbf{B}$ because it doesn't change within the iron):
$$\mathbf{M} = \chi_{m} \mathbf{H}$$
while
$$\mathbf{B} = \mu \mathbf{H},$$
where $\mu$ is the permeability of iron.
Putting it into our formula above, for a small chunk of our iron we have we have $$\mathbf{F} = \mu \chi_{m} \nabla (H^2) dV$$ Since we want the force to be constant in a single direction, we end up with: $$\frac{\partial}{\partial z}(H^2) = constant$$ which implies $$ H = \sqrt{Cz + f(x,y)}$$
But we have one further hurdle to clear. <PERSON>'s equations require that $\nabla \cdot \mathbf{H} = 0$, so we must have $f$ chosen so that
$\frac{\partial f}{\partial x} + \frac{\partial f}{\partial y} + C = 0$,
where C is a constant relating to the acceleration desired and the permeability of iron. There are many such functions that exist, so so far our task is possible. But we have one more hangup: unless you want this field caused by currents permeating all of space (which is rather unphysical), Ampere's Law requires that the curl of $\mathbf{H}$ also be zero in the vacuum through which the Earth is traveling. It turns out that this requires $\frac{\partial f}{\partial x} = 0$ and $\frac{\partial f}{\partial y} = 0$, something incompatible with our previous constraints on f.
Now, I must point out that all of this hinges on the assumption that our magnetic field only points in the z direction for all space, and that's mostly because I'm lazy and this answer is already very long. I suspect that a similar answer would turn up even if I did take into account a more general case, or at least it would lead to problems with stability of the Earth's motion. 418