What unused backing vocals?
I don't recall ever hearing any on the multitracks, and I'm pretty sure I don't hear any here. Would be a nice surprise if I've missed something.
Yes there is, it's the acapella "She's a Killer Queen" part originally recorded to be put under the guitar solo but eventually muted in the final mix.
It differs from the first "She's a Killer Queen" at 1:06 in the original song as in the first part the vocals transpose up at the end of "Queen", the unused part doesn't.
Added a soundclip of the vocal part unmuted in the solo and a comparison of both the first and second use of the lyric.