I may be wrong, but I believe it was Spike Edney. He did feature on the Live In Budapest video.
According to Wikipedia: "His work with Queen included mostly keyboards (his main instrument), plus rhythm guitar and backing vocals. He accompanied the band on their tours since 1984."
http://en.wikipedia.org/wiki/Spike_Edney
who played piano/guitar with Queen in Budapest alongside the usual band in 1986?
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