My take on the Powerslave backing vox is:
It's highly improbable that they are single note samples of Bruce's original recordings being played on a keyboard. For various reasons.*
I don't like it either, but I'm pretty sure it's just a track. As
@matic22 said, the track either starts at the beginning of the song or gets triggered by whomever when these voices enter.
But EVEN IF these voices were samples played on a keyboard - which they certainly aren't IMO - I would dislike that to almost the same degree.
*Who should be the keyboard tech master in the Maiden camp, that not only sampled every single note off of every single backing voc track (I wonder if these even still exist individually), but also programmed them so that they float organically as you described (Rudess) etc.? This just doesn't sound like Maiden; pragmatically, they would just never do that. What would even be the point, when you can just run a track? Cheating is cheating. AND it just sounds like the unaltered original vocals.