I think the sheer fact it's a third night bodes well. I'm sure we're all familiar with U2's tendency to deviate a bit from the norm on their final night in cities with 3+ shows. The fact there might be three shows to play the song at means it's got more chance of being played than if there were only one. I still don't expect it on opening night, but I do not think it is out of the question for the second and third nights. I'm not going to say that I think it's ONLY possible on the rumoured third night, but if it's going to happen on any night, that's the one. Opening night will obviously be rough and the band will try to stick to an anticipated standard set. Second night will be working out the kinks after analysing opening night. Third night, they'll probably be more relaxed, more willing to play something just for fun, and to toast a good start to the tour.
The odds unfortunately are still stacked against us. The fact it's the start of the tour; the fact Spanish Eyes has not been played in a stadium since 1987. But I now don't think cautious optimism is misplaced. U2 DO like to celebrate 3+ night stands in a city, usually with Party Girl but sometimes with other treats too. So let's hope the third show is confirmed!