What is it about the packaging that would create that taste?
Simplest explanation is that the beer itself was different...different batches, or more likely given BMC's ridiculously good consistency, different ages. Longer time on the shelf for one batch perhaps. I mean, sitting in the back of a store refrigerator for weeks is a sort of cold conditioning....

The answer for me would be to add more beers to the group. It would suck having to buy 8 different cases of what's generally considered not great beer, so Denny's idea of doing it in a BJCP class sounds great. If you can increase the number of trained testers, and increase the sample set significantly of beers, then if there is a difference you should see a nice pattern emerge. And all the different viewpoints should help define the differences too...we all use different language to describe taste sensations. Seems like "metallic" is off the table at least, at least by this test.