I think it was the other way around ie Katrina Benefit first, in Sept 2005. This was also long before the decision to cut the track was made. It was a taped for TV performance with no audience, so it would have been hard for them to judge "crowd" reaction at the time. Less than a month later, they asked her back onstage at MSG at a show and fuck yeah, it went off! I was there, it was amazing, moving, the crowd ate it up, a real highlight of the tour. Then they did it a few times later whenever they were in the same city. Finally, they recorded it using some studio vocals of hers and some stage recordings of One from the Vertigo tour, with some mixing and editing. Lastly (I think?) they did the video for it.
But your point is completely valid, there was likely a point where they all looked at each other and went 'wow, that was truly special, that needs to be cut properly' - it was totally a spontaneous organic thing that grew out of two artists with mutual respect for one another trying to do a good thing and show a UNITED FRONT IE ONE (in a time where we had Kanye saying GB doesn't care about black people, tensions high etc) and the notion that it was some sort of cash grab or cross marketing attempt is about as absurd as it gets. From what I recall they both happened to be in Toronto when the Katrina benefit was announced (unless of course, Paul McG plotted the hurricaine too...along with what else, 9/11 to boost ATYCLB sales?
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It was arguably even more organic than The Wanderer, a song that Bono imagined Johnny Cash singing and went thru several iterations or at least working titles, finally to invite Cash to the studio specially to sing the song. That's about as contrived as it gets, no? But not even I would be so foolishly cynical to suggest that U2 teamed up with Cash to try to win over die hard country music fans or boost record sales off the back of Cash's popularity with his fans. Good grief!
