Is the ammonium substituent (-NH3+) really meta-directing in electrophilic substitution?
how is anilinium ion meta directing for electrophiles?


Actually, anilinium is not meta directing (I know it is often taught that way), but rather it inductively deactivates the entire aromatic ring. 

To explain the electrophilic substitution pattern observed with any aromatic system we must consider both resonance and inductive effects.

Resonance Effects: You are correct that you can't delocalize the positive charge on the anilinium nitrogen into the benzene ring through resonance.  As you point out, resonance structures with 5 bonds to nitrogen aren't allowed.  



Therefore resonance effects are not involved in explaining the reactivity of the anilinium ion towards electrophilic aromatic substitution.

Inductive Effects: Inductively we expect the anilinium nitrogen to strongly withdraw electron density from the ortho position since it is closest to the positively charged nitrogen, this makes the ortho position the most deactivated.  Inductive effects die off quickly with distance, so we would expect the meta position to also be deactivated, but less so than the ortho position.  Finally, we would still expect some deactivation at the distant para position, but slightly less than we saw at the meta position. Overall, in terms of reactivity we would expect para > meta > ortho, with the difference between para and meta being much smaller than the difference between meta and ortho due to the effect distance has on inductive electron withdrawal.

If we examine the nitration of aniline in strong acid ($\ce{HNO3}$, $\ce{H2SO4}$), under these conditions the strong acid will completely protonate the aniline and reaction will proceed through the anilinium ion.  We see that, as expected, there is very little ortho product and slightly more para product than meta product.  This is pretty much as we expected and is consistent with the electron withdrawing inductive effect of the positive anilinium nitrogen.