I ran into an old friend of mine who was very connected with the music scene at NAMM a couple of years back and he did confirm there was a list of out there. Not sure if the one posted was it, but he said that the names were big names. I pressed him on it and he said he wouldn't name names (except for Blaze), but he said for sure he knew of 4.
#1 - Former singer of Helloween. He said he was always Steve's and Bruce's first choice because of his vocal range and his attitude (he is known as very nice person to deal with). Said the gig would've been Michael Kiske's, but he was sooo burnt out by what the other guys in Helloween did to him (and Ingo, the drummer's eventual suicide), he made it clear he would never sing metal again. That it killed both Kiske and Harris that Kiske didn't want to do it.
#2 - Former singer who replaced a legend from one of biggest metal bands of all time. I took this to mean Ronnie James Dio. Said there was communication between the singer and Harris, but didn't want to be seen as the guy who keeps replaceing legendary lead singers.
#3 - Former lead singer of IM. Since Steve wanted to go in a darker and heavier sounding direction, he thought about bringing back Di'Anno. They all always remained friends (including w/ Bruce), but the band thought there was no way he could've had the range to do Bruce's stuff and the band was still weary if he could handle the grind of touring w/o turning back to his old demons that got him kicked out of the band in the first place.
#4 - Blaze. Mostly because Steve thought he could mold him into sounding like Di'Anno and he could get away with singing Bruce's stuff. Steve was wrong.
What brought Bruce and Adrian back was Steve listening to Accident of Birth and Chemical Wedding. Bruce and Adrian did Maiden better than Maiden did Maiden.
He never told me how he knew, but he was very connected, so I took his word.