I just went back through the Ayreon facebook page and compiled the list of guests he's revealed so far. I believe he's done with vocalists and keyboardists, but there might be a couple more instrumental guests left to reveal, but here's what he's revealed so far (in reverse order with the latest first):
Guest Vocalists:
Russell Allen (Symphony X) (NOT confirmed but most likely)
John Jaycee Cuijpers
Wilmer Waarbroek (Ayreon)
Brandon Yeagley (Crobot)
Damian Wilson
Dan Swano
Michael Mills (Toehider)
Ross Jennings (Haken)
Will Shaw (Heir Apparent)
Jeff Scott Soto (Sons Of Apollo/Malmsteen/Journey)
Allesandro Del Vecchio
Mike Andersson (Tungsten)
Joe Lynn Turner (Rainbow/Deep Purple)
Brittney Slayes (Unleash The Archers)
Marcela Bovio
Tony Martin (Black Sabbath)
Irene Jansen
Roy Khan
Floor Jansen
Guest Instrumentalists:
Jens Johansson (Keyboard, Rainbow/Yngwie/Stratovarius)
Arjen Lucassen (Guitar, some random stuff)
Joost van den Broek (Keyboard, Ayreon)
Joel Hoekstra (Guitar, Whitesnake/Cher/TSO)
Marcel Singor (Guitar, Kayak)
Allesandro Del Vecchio (Keyboard)
Adrian Vandenberg (Guitar, Whitesnake)
Ron Bumblefoot Thal (Guitar, Sons Of Apollo, Guns N' Roses)
Lisa Bella Donna (Moog Synthesizers)
Michael Romeo (Guitar, Symphony X)
Timo Somers (Guitar, ex-Delain)
Steve Vai (Guitar)
A WHOOPING nineteen guest vocalists, over the course of 11 songs (well, 22 since there are two versions of each song). This is definitely a star-studded album, but I'm hoping it'll stand up on its own and be a strong one. I've only listened to the first single, so I'm probably gonna leave the rest of it as a surprise until I get my artbook in.
-Marc.