Reversing current also reverses, to some non-unity extent,
the material drift of electromigration. So an AC EM limit is
not infinite, but 10X DC is a conservative rule of thumb. A
single narrow pulse (one time event, like ESD) could go 100X
but you should back up any such figuring with an ohmic
volume adiabatic heating calc and require the risen temp
to remain under the metal liquidus (~600C for aluminum).
1E5 DC, 1E6 AC A/cm2 are good conservative current
densities.
However one thing you do not know without the help of
the foundry or an FA lab, is what the true worst case
cross section is (over-etch, step coverage, film thickness
control limits).