I remember the days when a Blue BMI card would get access at the United Red Carpet Clubs.
In similar times, I would often pay USD 25 for a day pass with USAirways if I had a multisector flights transiting CLT or PHL or LGA. or PIT.
In some cases (LGA-SJU) you would have to fly to PIT or PHL then on to CLT before going to SJU so it was well worth it.
Then my BAEC Silver got me lounge access at the USAir Clubs which was quite good.
A couple of times, I have asked lounge staff if I could bring a companion in who was flying another airline. BMI in Dublin allowed me one when I was flying Business as a Silver Card member when a friend was flying Ryanair.
BA also allowed me to take 3 guests into the lounge at T4 though this was at 5am on a Saturday morning on the first flight to Amsterdam. The lounge host cited the fact of getting a good cup of coffee at stupid o’clock was good enough reason to be flexible.
Obviously in both cases had we been advised it was not possible we would have respected the rules and gone somewhere else.