Though, umm, I think the hole drilling was to tell some drives that it was a double density 3.5 rather than a single density (170kb to 340kb, or something similar to that..) It was, IIRC, before the 740kb and 1.44mb discs came out. Couldn't do it to flip the disc over (first problem is that there wouldn't be anything to move the gate out of the way to reach the media that way..)
You might (and probably are right) in the double density thingy. I just remember reading it somewhere and trying it ... after I found it to be true, everyone wanted me to take their disks home and repeat my success. Spent several weekends drilling disks. LOL!