What's his reason for going to Metropolis to help then? The first two points wouldn't matter, cause they don't know that he isn't metahuman or part of the League. Everybody lies at some point in their life. There's also the possibility of a dirt rode or trail he could follow, or just ditch his car somewhere and go in on foot. Most of the troops would be on the front line making sure nothing comes out of Metropolis. 

EDIT: Or [@Weird Tales]'s St. William could 'feel' his demonic presence and go investigate this guy in an old muscle car who's trying to get into the danger zone...

EDIT #2: Or you could just exploit a hole in their security outside of Metropolis, which would be more likely. Suspension of disbelief and all...

~KL~