All right just for the record I've isolated it to an OS level issue. I even became the apache user and executed the spool file from /var/spool/at and it worked! But in root's mail I'm getting (consistently when the job is run):

This account is currently not available.
[download]

Now I may not understand exactly what that means, but it seems reasonable to believe that that's the barrier I'm running up against. I do appreciate y'alls input, and I learned a few things so your time was not wasted :-)