Not sure if this is helpful or not, but my husband (green card holder) and I just travelled back from Germany via Dublin. My husband always provides his green card and extension letter straight away - the Aer Lingus staff who checked us into our flight in Frankfurt barely looked at the green card and extension letter. Once we arrived in Dublin, we had to go to an Aer Lingus desk inside the terminal to get boarding passes to the US. They did briefly look at the green card and extension letter there. Not sure what would happen if it were a copy. Then we had to pass through US Customs in Dublin. I do think they seemed a little more relaxed than in the US. The immigration officer noted the green card was expired and then said something like "ok so you have an extension letter here..." and asked us "do you know how long this extension letter is good for?" lol (obviously yes we are aware...), so I'm not sure he had seen them very often but seemed very relaxed about it. Again, not sure what would have happened had it been a copy of the real extension letter.
Interested to see how your experience goes because this is basically something I stress about every time we travel. Good luck!
