That does sound like an interesting exercise. I will have to try it sometime.As Dr. Toole said, generally a few inches will do the trick, and a foot is generally more than enough. The actual minimum distance varies widely with the speaker, though.
It's actually not too hard to hear the effects of this. Get to the listening position and get someone else to hold something flat and rigid to serve as a "wall" (a small piece of plywood or a clipboard or something). Play something with a lot of content near the port tuning frequency, turn up the volume so the speaker is working hard, and have the other person gradually move the "wall" closer to the port. When it gets close enough to the port the airflow will be affected and it'll change the response of the speaker—this is usually quite audible, and it's kind of a fun exercise. Some speakers you can hear it quite clearly even 6-7" away from the speaker, and for some speakers you won't hear much of anything change until the "wall" is almost in contact with the speaker.
I wonder why many "audiophiles" insist that rear-ported speakers need a huge amount of space behind them to sound good, sometimes several feet. I guess maybe moving the speakers around changes the interactions with the room and they attribute those changes to the distance between the wall and the rear port.