Ric,
A couple of thoughts about this artifact. Once in my misspent youth we decided that it would be fun to toss a 1/4 stick of dynamite in my mother's newspaper mailbox (the plastic kind). While the explosion was quite intense and the mailbox displayed an amazing amount of hangtime, the part I remember the most was the pole still standing and swaying back and forth, with the bolt that had held the bracket to the box still intact, complete with washer. Why this is noteworthy is that when I looked at the bracket itself, it was perfectly fine as well, the bracket having been popped right over the washer and nut with no damage to any of the parts. The box was not nearly so fortunate. Depending on the force applied, I think it quite possible for an aluminum sheet to pop off of rivets without distorting the sheet. Also, keep in mind that rivet heads tend to be very thin. I can envision the heads bending up or shearing from the shaft before the sheet would distort. Lastly, in the location where this panel would have been if it were part of the plane, is there any structure that would tend to concentrate the hydraulic force in such a manner to create the artifact as we see it? I'm picturing an access panel in the floor or similar that, if open, could allow a sudden blast of water in to create the artifact.
LTM- Who is still mad about the wanton destruction to this day.