Maybe it's just semantics, but "how"? You can't even get a boarding pass for the return flight without first going through HM's Immigration at Heathrow (dunno about Japan or Peru) to get to the ticket counter where it's issued. You certainly couldn't have done so at Shannon and they wouldn't let us stay on the plane without one...

If by "entering" you mean going outside the airport, "sure"; no need to tour the Dublin suburbs out by the airport. By "entering" I mean setting foot on their soil, even if only in an airport.

I've told US Immigration - without any questions from them - that I did MR's for the miles. But I had to go somewhere - in another country - in order to get in a line to stand in front of an Immigration agent to tell him or her that.

I may be mistaken, but seem to recall not being allowed access to the LHR Admiral's Club without a boarding pass when I did just that in London: I wanted it issued in the club after deplaning - those halls are soooo long - but was told I had to go through Immigration and then return.