On dimeadozen there is a Guns 'n' Roses concert from Wembley in 13th June 1992 that says it features Brian on guitar for 2 tracks (Tie Your Mother Down & We Will Rock You)
I'm not a big fan of Buns & Toasties so I won't be downloading it but for anyone who wants it (and has access to dimeadozen) then it is there.
Concert posted at 28th May at about 4am according to my screen but don't know if different times show for different users around the world on that site