I try to make the complete list of Queen's collaborations to each other solo projects, but there questions still are. So:
Star Fleet Project (Brian) - Roger (back-vocal on 'Star Fleet')
Strange Frontier (Roger) - Freddie (back-vocal on 'Killing Time' and I'm sure co-author because of middle of the song)
John (bass on 'It's An Illusion', remixed 'I Cry For You' for 12")
Mr.Bad Guy (Freddie) - Brian (lead-guitar on 'She Blows Hot And Cold', released only on the FM Box Set)
The Great Pretender (Freddie) -....? OIQFC says Roger sings back-vocal, but I can't say that I'm sure. It could be Peter Straker or Freddie by himself. Roger's video appearance is not evidence, so some help needs.
Shove It (Roger) - Freddie (lead vocal on 'Heaven For Everyone' and helped with 'Shove It', but how? Mixed all those Queen samples?)
Brian (lead guitar on 'Love Lies Bleeding')
John (History says he was there, but where exactly? I don't think he plays bass as it's stated, because I can't see reason - his bass guitar is not like the Red Special or unique Freddie's voice, so why Peter Noone couldn't played? Maybe some mixing?)
Barcelona (Freddie) - John (bass on 'How Can I Go On')
Back To The Light (Brian) - John (bass on 'Nothing But Blue')
That's all! But questions are left...