I never made the lineup, but I did this once, a loooooong time ago (1979, with approval of the local University police) ... I shared a locked bike shed for 5 years with about 30 others, and it had been broken into perhaps 3 times a few years earlier, leaving a smaller frame Raleigh International with nothing but the bottom bracket and frame left...and there it sat, locked to the rack. The U police put notices on this and other abandoned bikes saying that they would be removed, so I asked if I could have this one.
I put a cut up styro box with the lock end at the bottom, and poured a styro cupful of liquid nitrogen, enough so it would pool round where the U goes through the lock end... after a minute or two of chilling, I just gave it a few really sharp blows with a carpenters hammer - no sledge needed. Broke right off...