I think the A-pillar bottom end is usually welded shut, perhaps a small weep or two if not filled with structural foam.
Would not be much of a fire wall if not sealed off anyway.
2012 Ford Focus chassis cutaway photo
2010 Audi A8 UK Body Structure | Boron Extrication
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The lower A-pillar (the large red part where the front door hinges wound be) is one of 25 castings in the body structure that interconnect the longitudinal member, the sill, the omega cross-member, the windshield crossmember, the roof frame and the suspension strut mount.
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Larger images in the link:
2010 Audi A8 UK Body Structure | Boron Extrication
Cutting a hole anywhere near an A-pillar just doesn't seem wise. However, if you can get yourself into a junkyard to see a stripped down wrecked chassis and do an investigative autopsy on it, I would be interested in seeing the results/photos.